[clang] [Clang][Sema] Warn when 'exclude_from_explicit_instantiation' attribute is used on local classes and members thereof (PR #88777)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@mikaelholmen I believe there are two uses of `exclude_from_explicit_instantiation` in libc++ which are responsible for all these warnings. I was assuming that someone more experienced with the libc++ side of things would handle it, but I can submit a patch that addresses these myself. 

https://github.com/llvm/llvm-project/pull/88777