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

[Aidan Goldfarb via cfe-commits]

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@@ -121,6 +121,16 @@ int NegativeShortTest[NegativeShort == -1 ? 1 : -1];
 enum Color { Red, Green, Blue }; // expected-note{{previous use is here}}
 typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type that does not match previous declaration}}
 
+// Enumerations with a fixed underlying type. 
+// https://github.com/llvm/llvm-project/issues/116880
+#if __STDC_VERSION__ >= 202311L && !__has_feature(c_fixed_enum)
+    #error c_fixed_enum should be set a feature in C23 mode
+#elif __STDC_VERSION__ < 202311L && !__has_extension(c_fixed_enum)
+    #error c_fixed_enum should be a language extension in <C23 mode
+#else
+    typedef enum : unsigned char { Pink, Black, Cyan } Color; // pre-c23-warning {{enumeration types with a fixed underlying type are a C23 extension}}
+#endif
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AidanGoldfarb wrote:

Updated. I chose to include 
`typedef enum : unsigned char { Pink, Black, Cyan } Color; // pre-c23-warning {{enumeration types with a fixed underlying type are a C23 extension}}` 
in `enum.c`, as I think we want to test for both the behavior of `__has_feature()` and `__has_extension()` as well as the actual definition of a typed enum. Please let me know if this is not a good idea! 

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