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

[Oleksandr T. via cfe-commits]

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@@ -822,6 +822,7 @@ def NSobjectAttribute : DiagGroup<"NSObject-attribute">;
 def NSConsumedMismatch : DiagGroup<"nsconsumed-mismatch">;
 def NSReturnsMismatch : DiagGroup<"nsreturns-mismatch">;
 
+def UnknownAttributeNamespaces : DiagGroup<"unknown-attribute-namespaces">;
 def IndependentClassAttribute : DiagGroup<"IndependentClass-attribute">;
 def UnknownAttributes : DiagGroup<"unknown-attributes">;
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a-tarasyuk wrote:

These test cases are definitely valuable, thanks! I've added them.

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