[clang] [Clang] raise extension warning for unknown namespaced attributes (PR #120925)

[Aaron Ballman via cfe-commits]

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@@ -179,6 +179,8 @@ def err_opencl_unknown_type_specifier : Error<
 
 def warn_unknown_attribute_ignored : Warning<
   "unknown attribute %0 ignored">, InGroup<UnknownAttributes>;
+def ext_unknown_attribute_ignored : Extension<
+  "unknown attribute %0 ignored">, InGroup<UnknownAttributes>;
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AaronBallman wrote:

Yes, we'd definitely want to support comma-delimited lists of these. I'm on the fence about the wording suggestion but not strongly opposed. I think the hardest part about my suggestion is that warning flags with `=` in them are poorly supported, so we'd need more tablegen machinery I think: https://github.com/llvm/llvm-project/blob/1594413d5edf6a47d4100cb6a2bc613cfbb92beb/clang/include/clang/Basic/DiagnosticGroups.td#L1036

https://github.com/llvm/llvm-project/pull/120925