[flang][openmp] Handle !$INCLUDE "foo" (bug #64128)

Detect and process INCLUDE lines that are guarded by OpenMP conditional
compilation markers (!$), when enabled.

Fixes https://github.com/llvm/llvm-project/issues/64128.

Differential Revision: https://reviews.llvm.org/D156759

flang/lib/Parser/prescan.cpp

@@ -106,7 +106,7 @@ void Prescanner::Statement() {
   case LineClassification::Kind::PreprocessorDirective:
     preprocessor_.Directive(TokenizePreprocessorDirective(), *this);
     return;
-  case LineClassification::Kind::CompilerDirective:
+  case LineClassification::Kind::CompilerDirective: {
     directiveSentinel_ = line.sentinel;
     CHECK(InCompilerDirective());
     BeginStatementAndAdvance();
@@ -118,22 +118,22 @@ void Prescanner::Statement() {
       }
       CHECK(*at_ == '!');
     }
+    std::optional<int> condOffset;
     if (directiveSentinel_[0] == '$' && directiveSentinel_[1] == '\0') {
-      // OpenMP conditional compilation line.  Remove the sentinel and then
-      // treat the line as if it were normal source.
-      at_ += 2, column_ += 2;
-      if (inFixedForm_) {
-        LabelField(tokens);
-      } else {
-        SkipSpaces();
-      }
+      // OpenMP conditional compilation line.
+      condOffset = 2;
     } else if (directiveSentinel_[0] == '@' && directiveSentinel_[1] == 'c' &&
         directiveSentinel_[2] == 'u' && directiveSentinel_[3] == 'f' &&
         directiveSentinel_[4] == '\0') {
-      // CUDA conditional compilation line.  Remove the sentinel and then
-      // treat the line as if it were normal source.
-      at_ += 5, column_ += 5;
-      if (inFixedForm_) {
+      // CUDA conditional compilation line.
+      condOffset = 5;
+    }
+    if (condOffset) {
+      at_ += *condOffset, column_ += *condOffset;
+      if (auto payload{IsIncludeLine(at_)}) {
+        FortranInclude(at_ + *payload);
+        return;
+      } else if (inFixedForm_) {
         LabelField(tokens);
       } else {
         SkipSpaces();
@@ -153,6 +153,7 @@ void Prescanner::Statement() {
       tokens.CloseToken();
     }
     break;
+  }
   case LineClassification::Kind::Source:
     BeginStatementAndAdvance();
     if (inFixedForm_) {

flang/test/Parser/OpenMP/cond-include.f90

@@ -0,0 +1,4 @@
+! RUN: %flang_fc1 -fdebug-unparse -fopenmp %s | FileCheck %s
+!CHECK: STOP "pass"
+!$ include "cond-include.inc"
+end

flang/test/Parser/OpenMP/cond-include.inc

@@ -0,0 +1 @@
+STOP "pass"