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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Note, I'm running this one through llvm-compile-time-tracker because of the lookup happening on every identifier. I'm not convinced that the "is this identifier a keyword in other language modes" code has good performance and wanted to double-check.

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