[diag] Silence `-Wfixed-enum-extension` in C23 (#68060)

The C23 standard supports enums with fixed underlying types (N3030 [1]),
so we shouldn't emit `-Wfixed-enum-extension` in C23 mode (since it's no
longer a Clang extension at that point).

[1]
https://thephd.dev/_vendor/future_cxx/papers/C%20-%20Enhanced%20Enumerations.html