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

[Aaron Ballman via cfe-commits]

AaronBallman 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?

Wow. My fingers are obviously not listening to my brain very well. :-D

I mean "warn me about unknown attribute, but not these particular unknown attributes." e.g., (fake warning group name)
```
// -Wwarn-about-unknown-attributes-except-these=foo,bar::baz
[[foo, bar::baz, quux]] int x; // warning: unknown attribute 'quux'
```

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