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

[via cfe-commits]

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@@ -121,6 +121,15 @@ 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
+    typedef enum : unsigned char { Pink, Black, Cyan } Color;
+#else
+    _Static_assert(__has_extension(c_fixed_enum), "Ensure language extension support for enumerations with a fixed underlying type in <C23");
+    typedef enum : unsigned char { Pink, Black, Cyan } Color; // expected-warning {{enumeration types with a fixed underlying type are a C23 extension}}
+#endif
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Sirraide wrote:

```suggestion
#if __STDC_VERSION__ >= 202311L && !__has_extension(c_fixed_enum)
#    error c_fixed_enum should be set 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}}
```
I think we usually test these via the preprocessor.

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