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

[Erich Keane via cfe-commits]

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@@ -7533,6 +7533,43 @@ A managed variable can be accessed in both device and host code.
   }];
 }
 
+def CUDAClusterDimsAttrDoc : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+In CUDA/HIP programming, the ``__cluster_dims__`` attribute can be applied to a kernel function
+to set the dimensions of a thread block cluster. ``__cluster_dims__`` defines the cluster size
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erichkeane wrote:

Can we quickly define in a couple of words what `thread block cluster` is here?

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