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

[Erich Keane via cfe-commits]

erichkeane wrote:

>That's how this already works in this patch:
AH! I missed that this was doing that.  Awesome!  Though I think the 'no' flag is the one that makes sense in a "turn it on for all BUT these". The flag as typed there isn't particularly useful (that is, ONLY warn me for the ones that I know about).

>I think what Corentin is asking for is a similar (follow-up?) feature that allows you to say "warn me about unknown attributes, but not of unknown attributes." that can take attributes with or without a namespace.

I'm not hearing a difference?

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