[NFC] [C++20] [Modules] Add tests for merging redefinitions in modules

Add tests for detecting redefinitions in C++20 modules. Some of these
may be covered by other tests. But more tests should be always good.

clang/test/Modules/redefinition-merges.cppm

@@ -0,0 +1,117 @@
+// Tests that redefinitions in different TUs could be merged correctly and the
+// redefinitions in the same TUs could be merged diagnosticed correctly.
+//
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -I%t %t/normal.cpp -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -I%t %t/M1.cppm -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -I%t %t/M2.cppm -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -I%t %t/M3.cppm -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -I%t %t/M.cppm -emit-module-interface -o %t/M.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t %t/Use1.cpp -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t %t/Use2.cpp -verify -fsyntax-only
+//
+//--- foo.h
+#ifndef FOO
+#define FOO
+inline void func() {}
+template <typename T>
+T templ_func(T t) { return t; }
+struct S {};
+template <class C>
+struct T { C c; };
+inline int v = 43;
+#endif
+
+// If we copy foo.h directly, there are other warnings.
+//--- redef.h
+#ifndef REDEF
+#define REDEF
+inline void func() {}
+template <typename T>
+T templ_func(T t) { return t; }
+struct S {};
+template <class C>
+struct T { C c; };
+inline int v = 43;
+#endif
+
+//--- normal.cpp
+#include "foo.h"
+#include "redef.h"
+
+// expected-error@* {{redefinition of 'func'}}
+// expected-error@* {{redefinition of 'templ_func'}}
+// expected-error@* {{redefinition of 'S'}}
+// expected-error@* {{redefinition of 'T'}}
+// expected-error@* {{redefinition of 'v'}}
+// expected-note@* 1+{{previous definition is here}}
+
+//--- M1.cppm
+// These declarations are in the same TU. The compiler should complain.
+module;
+#include "foo.h"
+#include "redef.h"
+export module M1;
+
+// expected-error@* {{redefinition of 'func'}}
+// expected-error@* {{redefinition of 'templ_func'}}
+// expected-error@* {{redefinition of 'S'}}
+// expected-error@* {{redefinition of 'T'}}
+// expected-error@* {{redefinition of 'v'}}
+// expected-note@* 1+{{previous definition is here}}
+
+//--- M2.cppm
+// These declarations are in the same TU and the redefinitions are in the named modules.
+// The compiler should complain.
+module;
+#include "foo.h"
+export module M2;
+#include "redef.h"
+
+// FIXME: The diagnostic message looks not so good.
+//
+// expected-error@* {{declaration of 'func' in module M2 follows declaration in the global module}}
+// expected-error@* {{declaration of 'templ_func' in module M2 follows declaration in the global module}}
+// expected-error@* {{redefinition of 'S'}}
+// expected-error@* {{redefinition of 'T'}}
+// expected-error@* {{declaration of 'v' in module M2 follows declaration in the global module}}
+// expected-note@* 1+{{previous definition is here}}
+// expected-note@* 1+{{previous declaration is here}}
+
+//--- M3.cppm
+// These declarations are in the same TU. The compiler should complain.
+export module M3;
+#include "foo.h"
+#include "redef.h"
+
+// expected-error@* {{redefinition of 'func'}}
+// expected-error@* {{redefinition of 'templ_func'}}
+// expected-error@* {{redefinition of 'S'}}
+// expected-error@* {{redefinition of 'T'}}
+// expected-error@* {{redefinition of 'v'}}
+// expected-note@* 1+{{previous definition is here}}
+
+//--- M.cppm
+module;
+#include "foo.h"
+export module M;
+export using ::func;
+export using ::templ_func;
+export using ::S;
+export using ::T;
+export using ::v;
+
+//--- Use1.cpp
+// These declarations are not in the same TU. The compiler shouldn't complain.
+// expected-no-diagnostics
+#include "foo.h"
+import M;
+
+//--- Use2.cpp
+// These declarations are not in the same TU. The compiler shouldn't complain.
+// expected-no-diagnostics
+import M;
+#include "foo.h"