[clang] [RFC][clang] Fix for regression #130917 (PR #132214)

[Dmitry Polukhin via cfe-commits]

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@@ -2572,7 +2572,7 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
   // Friend function defined withing class template may stop being function
   // definition during AST merges from different modules, in this case decl
   // with function body should be used for instantiation.
-  if (isFriend) {
+  if (isFriend && D->hasOwningModule()) {
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dmpolukhin wrote:

@ChuanqiXu9 sorry I'm not sure I fully understand your idea about ExternalASTSource. Are you suggesting to extend `ASTReader::DefinitionSource` to also keep this information there? Now `ASTReader::DefinitionSource` maps to `bool` but I can change value to a struct with two flags (previous DefinitionSource and ThisDeclarationWasADefinition), I think we don't need to store it in PCM file. Does it sound like the right approach?

https://github.com/llvm/llvm-project/pull/132214