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

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@@ -308,6 +308,10 @@ EXTENSION(datasizeof, LangOpts.CPlusPlus)
 
 FEATURE(cxx_abi_relative_vtable, LangOpts.CPlusPlus && LangOpts.RelativeCXXABIVTables)
 
+//Fixed enum feature and extension, to be relocated in this file
+FEATURE(c_fixed_enum, true)    
+EXTENSION(c_fixed_enum, true)  
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Sirraide wrote:

Haven’t looked at the features code in a bit, but does this need to be both a FEATURE and an EXTENSION?

CC @AaronBallman 

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