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

[Nick Desaulniers via cfe-commits]

https://github.com/nickdesaulniers approved this pull request.

LGTM; though I have a strong preference to get bugs on file and linked to regarding:
- issues with non-itanium mangling
- issues with namespaced identifiers.

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