[C++20] [Modules] Don't diagnose duplicated declarations in different modules which is not in file scope

Close #126373

Although the root problems should be we shouldn't place the friend
declaration to the incorrect module, let's avoid bleeding the edge by
stoping diagnosing declarations not in file scope.

(cherry picked from commit 569e94f)

Author: ChuanqiXu9

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3751,6 +3751,13 @@ void ASTDeclReader::checkMultipleDefinitionInNamedModules(ASTReader &Reader,
   if (D->getFriendObjectKind() || Previous->getFriendObjectKind())
     return;
 
+  // Skip diagnosing in-class declarations.
+  if (!Previous->getLexicalDeclContext()
+           ->getNonTransparentContext()
+           ->isFileContext() ||
+      !D->getLexicalDeclContext()->getNonTransparentContext()->isFileContext())
+    return;
+
   Module *M = Previous->getOwningModule();
   if (!M)
     return;

clang/test/Modules/pr126373.cppm

@@ -0,0 +1,34 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/module1.cppm -emit-module-interface -o %t/module1.pcm
+// RUN: %clang_cc1 -std=c++20 -fmodule-file=module1=%t/module1.pcm  %t/module2.cppm \
+// RUN:     -emit-module-interface -o %t/module2.pcm
+// RUN: %clang_cc1 -std=c++20 %t/module2.pcm -fmodule-file=module1=%t/module1.pcm \
+// RUN:     -emit-llvm -o - | FileCheck %t/module2.cppm
+
+//--- test.h
+template<typename T>
+struct Test {
+  template<typename U>
+  friend class Test;
+};
+
+//--- module1.cppm
+module;
+#include "test.h"
+export module module1;
+export void f1(Test<int>) {}
+
+//--- module2.cppm
+module;
+#include "test.h"
+export module module2;
+import module1;
+export void f2(Test<float>) {}
+
+extern "C" void func() {}
+
+// Fine enough to check the IR is emitted correctly.
+// CHECK: define{{.*}}@func