[clang] [Sema] Mark alias/ifunc targets used and consider mangled names (PR #87130)

[Fangrui Song via cfe-commits]

MaskRay wrote:

I think I've addressed all comments and changed the patch to rebase on the improved tests.

@AaronBallman @tstellar Are you happy with the current version?

https://github.com/llvm/llvm-project/pull/87130