[clang] [clang][sema] Add support and documentation for `__has_extension(c_fixed_enum)` (PR #117507)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM aside from some suggested edits to the documentation. Mostly just adding full stops to the end of sentences, and a mention about the behavior of `__has_extension(c_fixed_enum)` in C23 and later.

https://github.com/llvm/llvm-project/pull/117507