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

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -fsyntax-only -Wunknown-attribute-namespace=foo,bar -std=c23 -verify %s
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AaronBallman wrote:

Can you also add a `RUN` line with `-x c++` to show we get the same behavior in C and C++?

Also, we may want to consider adding a `FileCheck` test (instead of a `-verify` test) so that we can validate the correct warning group is emitted after the diagnostic. e.g., unknown namespaces should get you `[-Wunknown-attribute-namespace]` while unknown attributes in a known namespace should still get `[-Wunknown-attributes]`

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