[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'
> > ```
> 
> Ah, hrm. I see much less value in that. How likely is it that you have an attribute in a 'known' namespace that is 'unknown' but valid? 

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


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