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

[Erich Keane via cfe-commits]

================
@@ -216,6 +216,9 @@ Improvements to Clang's diagnostics
 - The fix-it emitted by ``-Wformat`` for scoped enumerations now take the
   enumeration's underlying type into account instead of suggesting a type just
   based on the format string specifier being used.
+- ``-Wfixed-enum-extension`` and ``-Wmicrosoft-fixed-enunm`` are no longer
----------------
erichkeane wrote:

```suggestion
- ``-Wfixed-enum-extension`` and ``-Wmicrosoft-fixed-enum`` diagnostics are no longer
```

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