[clang] [C] Diagnose use of C++ keywords in C (PR #137234)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Adding @rjmccall because this touches on Objective-C `@` keyword handling. (Note, maybe we want `-Wc++-compat` to be disabled in Objective-C entirely and we should have `-Wobjc++-compat` instead? `[foo bar];` is not valid C++, but I doubt anyone wants that diagnosed in Objective-C code, so it's a bit unclear what the diagnostic group means for ObjC.)

https://github.com/llvm/llvm-project/pull/137234