[clang] [Clang][HIP][CUDA] Add `__cluster_dims__` and `__no_cluster__` attribute (PR #156686)

[Artem Belevich via cfe-commits]

Artem-B wrote:

> > > I think the declspec spellings aren't done yet
> > 
> > 
> > I think that is also part of the "established nomenclature" that @Artem-B was referring to.
> 
> The difference is (AFAIK) that Clang actually ALWAYS supports the double-underscore start/end spellings by adding them to the beginning/end of the existing name. So I'm saying you shouldn't need them in the Attr.td file, as they will be auto-added. You can document the "established nomenclature"/use it anyway, but the Attr.td shouldn't have it AFAIK

The user-facing double-underscore attribute variants are preprocessor macros added by CUDA headers. The attributes themselves do not have them. At least that's the case for CUDA. I'm not sure about HIP.

https://github.com/llvm/llvm-project/pull/156686