[clang] [clang] Diagnose the _BitInt suffix as an extension in GNU C modes (PR #217815)

[Ambrose Leeb via cfe-commits]

https://github.com/Sirraide commented:

The implementation of the change in and of itself is fine, but I'm not we care about matching GCC wrt some extension warnings being disabled in GNU mode if `-pedantic` is passed—I personally think that that’s weird and that they _shouldn’t_ be.

CC @AaronBallman do we have any precedent for this?

https://github.com/llvm/llvm-project/pull/217815