[clang] [Clang] diagnose unknown attribute namespaces (PR #138519)

[Oleksandr T. via cfe-commits]

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@@ -385,6 +385,8 @@ related warnings within the method body.
 - Clang now disallows the use of attributes applied before an
   ``extern template`` declaration (#GH79893).
 
+- Clang now diagnoses unknown attribute namespaces
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a-tarasyuk wrote:

@erichkeane I’ve updated PR to use existing diagnostic with scope 

https://github.com/llvm/llvm-project/pull/138519