[clang] [Clang][NFC] Update diagnostic handling for C2y compatibility in parser (PR #209920)

[via cfe-commits]

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@@ -169,12 +169,7 @@ def ext_c2y_generic_with_type_arg : Extension<
 def warn_c2y_compat_generic_with_type_arg : Warning<
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Sirraide wrote:

Yes, I only updated the ones in `DiagnosticSemaKinds.td` in my patch; we should do this file as well

https://github.com/llvm/llvm-project/pull/209920