Close https://github.com/llvm/llvm-project/issues/138558
The compiler failed to understand the redeclaration-relationship when
performing checks when MergeFunctionDecl. This seemed to be a complex
circular problem (how can we know the redeclaration relationship before
performing merging?). But the fix seems to be easy and safe. It is fine
to only perform the check only if the using decl is a local decl.
