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

[Aaron Ballman via cfe-commits]

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@@ -1998,6 +1998,9 @@ value, is ``unsigned char``.
 Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
 underlying types is available in Objective-C.
 
+Use ``__has_extension(c_fixed_enum)`` to determine whether support for fixed
+underlying types is available in C.
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AaronBallman wrote:

We should probably update the wording here to mention `__has_feature` and language version behavior.

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