[clang] [Sema] Don't drop weak_import from a declaration that follows a declaration directly contained in a linkage-specification (PR #85886)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

The updated patch checks that the decl isn't `DeclarationOnly` to make it clear that we are looking for all definitions although we know we won't find any full definitions.

https://github.com/llvm/llvm-project/pull/85886