[clang] [APINotes] Diagnose invalid Where.Parameters selectors (PR #209408)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun commented:

Overall,  looks great! Some further comments below but I think we can merge this soon!

I am wondering if `ShowInSystemHeader` should be set to 1 for `warn_apinotes_message` because APINotes are often used for system headers.

But this could be a separate PR. 

https://github.com/llvm/llvm-project/pull/209408