[lldb][TypeSystemClang][NFCI] Factor completion logic for individual types out of GetCompleteQualType (#95402)

This patch factors out the completion logic for individual clang::Type's
into their own helper functions.

During the process I cleaned up a few assumptions (e.g., unnecessary
if-guards that could be asserts because these conditions are guaranteed
by the `clang::Type::TypeClass` switch in `GetCompleteQualType`).

This is mainly motivated by the type-completion rework proposed in
https://github.com/llvm/llvm-project/pull/95100.

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -2574,6 +2574,128 @@ TypeSystemClang::GetDeclContextForType(clang::QualType type) {
   return nullptr;
 }
 
+/// Returns the clang::RecordType of the specified \ref qual_type. This
+/// function will try to complete the type if necessary (and allowed
+/// by the specified \ref allow_completion). If we fail to return a *complete*
+/// type, returns nullptr.
+static const clang::RecordType *GetCompleteRecordType(clang::ASTContext *ast,
+                                                      clang::QualType qual_type,
+                                                      bool allow_completion) {
+  assert(qual_type->isRecordType());
+
+  const auto *tag_type = llvm::cast<clang::RecordType>(qual_type.getTypePtr());
+
+  clang::CXXRecordDecl *cxx_record_decl = qual_type->getAsCXXRecordDecl();
+
+  // RecordType with no way of completing it, return the plain
+  // TagType.
+  if (!cxx_record_decl || !cxx_record_decl->hasExternalLexicalStorage())
+    return tag_type;
+
+  const bool is_complete = cxx_record_decl->isCompleteDefinition();
+  const bool fields_loaded =
+      cxx_record_decl->hasLoadedFieldsFromExternalStorage();
+
+  // Already completed this type, nothing to be done.
+  if (is_complete && fields_loaded)
+    return tag_type;
+
+  if (!allow_completion)
+    return nullptr;
+
+  // Call the field_begin() accessor to for it to use the external source
+  // to load the fields...
+  //
+  // TODO: if we need to complete the type but have no external source,
+  // shouldn't we error out instead?
+  clang::ExternalASTSource *external_ast_source = ast->getExternalSource();
+  if (external_ast_source) {
+    external_ast_source->CompleteType(cxx_record_decl);
+    if (cxx_record_decl->isCompleteDefinition()) {
+      cxx_record_decl->field_begin();
+      cxx_record_decl->setHasLoadedFieldsFromExternalStorage(true);
+    }
+  }
+
+  return tag_type;
+}
+
+/// Returns the clang::EnumType of the specified \ref qual_type. This
+/// function will try to complete the type if necessary (and allowed
+/// by the specified \ref allow_completion). If we fail to return a *complete*
+/// type, returns nullptr.
+static const clang::EnumType *GetCompleteEnumType(clang::ASTContext *ast,
+                                                  clang::QualType qual_type,
+                                                  bool allow_completion) {
+  assert(qual_type->isEnumeralType());
+  assert(ast);
+
+  const clang::EnumType *enum_type =
+      llvm::cast<clang::EnumType>(qual_type.getTypePtr());
+
+  auto *tag_decl = enum_type->getAsTagDecl();
+  assert(tag_decl);
+
+  // Already completed, nothing to be done.
+  if (tag_decl->getDefinition())
+    return enum_type;
+
+  if (!allow_completion)
+    return nullptr;
+
+  // No definition but can't complete it, error out.
+  if (!tag_decl->hasExternalLexicalStorage())
+    return nullptr;
+
+  // We can't complete the type without an external source.
+  clang::ExternalASTSource *external_ast_source = ast->getExternalSource();
+  if (!external_ast_source)
+    return nullptr;
+
+  external_ast_source->CompleteType(tag_decl);
+  return enum_type;
+}
+
+/// Returns the clang::ObjCObjectType of the specified \ref qual_type. This
+/// function will try to complete the type if necessary (and allowed
+/// by the specified \ref allow_completion). If we fail to return a *complete*
+/// type, returns nullptr.
+static const clang::ObjCObjectType *
+GetCompleteObjCObjectType(clang::ASTContext *ast, QualType qual_type,
+                          bool allow_completion) {
+  assert(qual_type->isObjCObjectType());
+  assert(ast);
+
+  const clang::ObjCObjectType *objc_class_type =
+      llvm::cast<clang::ObjCObjectType>(qual_type);
+
+  clang::ObjCInterfaceDecl *class_interface_decl =
+      objc_class_type->getInterface();
+  // We currently can't complete objective C types through the newly added
+  // ASTContext because it only supports TagDecl objects right now...
+  if (!class_interface_decl)
+    return objc_class_type;
+
+  // Already complete, nothing to be done.
+  if (class_interface_decl->getDefinition())
+    return objc_class_type;
+
+  if (!allow_completion)
+    return nullptr;
+
+  // No definition but can't complete it, error out.
+  if (!class_interface_decl->hasExternalLexicalStorage())
+    return nullptr;
+
+  // We can't complete the type without an external source.
+  clang::ExternalASTSource *external_ast_source = ast->getExternalSource();
+  if (!external_ast_source)
+    return nullptr;
+
+  external_ast_source->CompleteType(class_interface_decl);
+  return objc_class_type;
+}
+
 static bool GetCompleteQualType(clang::ASTContext *ast,
                                 clang::QualType qual_type,
                                 bool allow_completion = true) {
@@ -2591,92 +2713,26 @@ static bool GetCompleteQualType(clang::ASTContext *ast,
                                  allow_completion);
   } break;
   case clang::Type::Record: {
-    clang::CXXRecordDecl *cxx_record_decl = qual_type->getAsCXXRecordDecl();
-    if (cxx_record_decl) {
-      if (cxx_record_decl->hasExternalLexicalStorage()) {
-        const bool is_complete = cxx_record_decl->isCompleteDefinition();
-        const bool fields_loaded =
-            cxx_record_decl->hasLoadedFieldsFromExternalStorage();
-        if (is_complete && fields_loaded)
-          return true;
+    if (const auto *RT =
+            GetCompleteRecordType(ast, qual_type, allow_completion))
+      return !RT->isIncompleteType();
 
-        if (!allow_completion)
-          return false;
-
-        // Call the field_begin() accessor to for it to use the external source
-        // to load the fields...
-        clang::ExternalASTSource *external_ast_source =
-            ast->getExternalSource();
-        if (external_ast_source) {
-          external_ast_source->CompleteType(cxx_record_decl);
-          if (cxx_record_decl->isCompleteDefinition()) {
-            cxx_record_decl->field_begin();
-            cxx_record_decl->setHasLoadedFieldsFromExternalStorage(true);
-          }
-        }
-      }
-    }
-    const clang::TagType *tag_type =
-        llvm::cast<clang::TagType>(qual_type.getTypePtr());
-    return !tag_type->isIncompleteType();
+    return false;
   } break;
 
   case clang::Type::Enum: {
-    const clang::TagType *tag_type =
-        llvm::dyn_cast<clang::TagType>(qual_type.getTypePtr());
-    if (tag_type) {
-      clang::TagDecl *tag_decl = tag_type->getDecl();
-      if (tag_decl) {
-        if (tag_decl->getDefinition())
-          return true;
-
-        if (!allow_completion)
-          return false;
-
-        if (tag_decl->hasExternalLexicalStorage()) {
-          if (ast) {
-            clang::ExternalASTSource *external_ast_source =
-                ast->getExternalSource();
-            if (external_ast_source) {
-              external_ast_source->CompleteType(tag_decl);
-              return !tag_type->isIncompleteType();
-            }
-          }
-        }
-        return false;
-      }
-    }
+    if (const auto *ET = GetCompleteEnumType(ast, qual_type, allow_completion))
+      return !ET->isIncompleteType();
 
+    return false;
   } break;
   case clang::Type::ObjCObject:
   case clang::Type::ObjCInterface: {
-    const clang::ObjCObjectType *objc_class_type =
-        llvm::dyn_cast<clang::ObjCObjectType>(qual_type);
-    if (objc_class_type) {
-      clang::ObjCInterfaceDecl *class_interface_decl =
-          objc_class_type->getInterface();
-      // We currently can't complete objective C types through the newly added
-      // ASTContext because it only supports TagDecl objects right now...
-      if (class_interface_decl) {
-        if (class_interface_decl->getDefinition())
-          return true;
+    if (const auto *OT =
+            GetCompleteObjCObjectType(ast, qual_type, allow_completion))
+      return !OT->isIncompleteType();
 
-        if (!allow_completion)
-          return false;
-
-        if (class_interface_decl->hasExternalLexicalStorage()) {
-          if (ast) {
-            clang::ExternalASTSource *external_ast_source =
-                ast->getExternalSource();
-            if (external_ast_source) {
-              external_ast_source->CompleteType(class_interface_decl);
-              return !objc_class_type->isIncompleteType();
-            }
-          }
-        }
-        return false;
-      }
-    }
+    return false;
   } break;
 
   case clang::Type::Attributed: