[clang] [RFC][clang] Handle friend function that was a definition but became only a declaration during AST deserialization (PR #132214)

[Dmitry Polukhin via cfe-commits]

================
@@ -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()) {
----------------
dmpolukhin wrote:

I would prefer ExternalASTSource approach because it won't add any additional memory overhead and we already have hashtable ASTReader::DefinitionSource. Access to the filed should be super cold so it won't slowdown things. I will update this PR soon.

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