[clang] [Clang] Add queryable feature 'boolean_vectors' for SIMD masks (PR #154227)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I clicked accept, but not sure I'm super in favor of the name? Maybe see if folks will bikeshed it for you? I'm a bit concerned it is too close to other things (vector is ambiguous of course).
> > Maybe:
> > `boolean_gnu_vector_type` and `boolean_ext_vector_type`
> > WDYT?
> 
> Yeah, I was wondering about that. I don't think we support the GNU vectors to be boolean yet so it can be a little confusing. Maybe just `ext_vector_type_boolean`?

ah, sure.  Then when/if we get `gnu` version we can do `gnu_vector_type_boolean`? Works for me and is 'specific' enough I'm no longer worried.  Make it so :) 

https://github.com/llvm/llvm-project/pull/154227