[clang] [diag] Silence `-Wfixed-enum-extension` in C23 (PR #68060)

[Erich Keane via cfe-commits]

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@@ -5019,7 +5019,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
         else if (getLangOpts().MicrosoftExt)
           Diag(ColonLoc, diag::ext_ms_c_enum_fixed_underlying_type)
               << BaseRange;
-        else
+        else if (!getLangOpts().C23)
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erichkeane wrote:

Do we want C23 mode handled higher up?  At least above `MicrosoftExt`?  What if the person is compiling in C23 mode with Microsoft Extensions, they'd get the above warning incorrectly.

https://github.com/llvm/llvm-project/pull/68060