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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Maybe `-Wkeyword-in-c++` or `-Wc++-keyword` would be a more concise name for the group?

I'm not tied to the name I picked, so either of these is fine by me. GCC doesn't split this into its own warning group, so we've got some latitude.

Any strong preferences?

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