[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?
> 
> Please file an issue for this:
> 
> > I added microsoftDemangle tests to show the current behavior.
> > Since the feature that demangles to the function name without parameters (f3 instead of f3(int)) appears to be missing, I cannot > address -Wunused-function false positives for microsoftDemangle with reasonable time complexity.
> 
> and file a second one to come back and fix this issue once we can demangle with parameters.

Created #88823 and #88825 

Thanks!

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