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

[Shilei Tian via cfe-commits]

shiltian 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).

> 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.

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