[clang] [clang] Add __nullptr as a keyword to C (PR #123119)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Please be sure to also update `clang/docs/ReleaseNotes.rst` so users know about the new functionality. Also, not signing you up for this work, but we really should document all of these keywords in `clang/docs/LanguageExtensions.rst`. Maybe it's worth starting with this one and hopefully we can backfill the rest in the future?

https://github.com/llvm/llvm-project/pull/123119