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

[John McCall via cfe-commits]

rjmccall wrote:

Yeah, triggering this at the preprocessor level is not going to let you do in properly in Objective-C, where keywords are effectively context-sensitive. If that's the only reasonable way to implement it, you should just turn it off in Objective-C completely.

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