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

[Aaron Ballman via cfe-commits]

================
@@ -163,6 +163,8 @@ FEATURE(c_atomic, LangOpts.C11)
 FEATURE(c_generic_selections, LangOpts.C11)
 FEATURE(c_static_assert, LangOpts.C11)
 FEATURE(c_thread_local, LangOpts.C11 &&PP.getTargetInfo().isTLSSupported())
+// C23 features
+FEATURE(c_fixed_enum, true) 
----------------
AaronBallman wrote:

```suggestion
FEATURE(c_fixed_enum, LangOpts.C23) 
```
This is only a feature of C23, it's an extension in other modes.

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