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

[Erich Keane via cfe-commits]

erichkeane wrote:

> But why do others, such as `CUDAGlobal`, still have `__` in `Declspec`? It indeed doesn't have it for `GNU`.

Likely: Less active review in the past.  Those should probably be removed.  Feel free to do it in a followup commit.

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