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

[Erich Keane via cfe-commits]

================
@@ -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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erichkeane wrote:

```suggestion
- Clang now diagnoses unknown attribute namespaces.
```

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