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

[Andrew Pinski via cfe-commits]

pinskia wrote:

>From my point of view this looks like a decent documentation of this extension. Note I filed the original bug because I noticed both GCC and clang didn't document this extension. (I filed GCC here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117689 which I got try fix next week).

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