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

[Nick Desaulniers via cfe-commits]

https://github.com/nickdesaulniers commented:

Progressing in the right direction and I'm in favor of fixing this issue in clang, with similar levels of caution as my fellow reviewers.

I think we can support Microsoft ABI mangling trivially here.

That we mangle globals with static linkage in `extern "C"` blocks looks to me like a bug in clang that should get fixed first BEFORE this lands.  Otherwise that seems like a massive compatibility issue.  Perhaps another approach would be to at least document this difference.  But then again, this behavioral difference is surprising, and I worry that it might be used to try to hide something.

The namespace lookup false positive is something that perhaps doesn't need to get fixed here; if we don't plan to remove that final false positive in the PR, then please file a bug @MaskRay and link to it from a TODO comment in the test case and/or sources.

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