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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > It isn't clear what these are, why/whether we want them, what their signature is, etc.
> 
> It is the support for [`__cluster_dims__` attribute](https://docs.nvidia.com/cuda/cuda-c-programming-guide/#thread-block-clusters).

ALL of this (including a summary that doesn't require a link) should be in the commit message and documentation.
> 
> > Additionally, we do not allow adding undocumented attributes anymore, so please make sure these get documented.
> 
> Noted. Will add some documents.
> 
> > And why can't these just use the other dimensions-attributes that we have (of which we have plenty).
> 
> I don't follow this one. Can you explain more? These are for different purposes.

are they?  There is nothing on this patch that indicates that (being a bit coy, the documentation should probably make it clear what the differences are/how it is used differently than all the other 'dim' based attributes).


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