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

[Erich Keane via cfe-commits]

erichkeane wrote:

>For example, we know about the gnu namespace, but there are a ton of attributes GCC supports but we do not.

Sure, but we've never needed/wanted that before for general attributes, right?  I am seeing THAT as particularly novel for this patch.  I would be open to reviewing that, but don't see it as necessary for this patch.

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