[clang] [clang] Alias iso9899:2024 to C23, update documentation (PR #138459)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman Hmm, should we also document `-std=c2y` and `-std=gnu2y`?

Yup, we should!

https://github.com/llvm/llvm-project/pull/138459