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

[Shilei Tian via cfe-commits]

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@@ -7532,6 +7532,25 @@ 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
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shiltian wrote:

I'd prefer to make it consistent with existing documentation wording, which does use `__`. A bulk update can be easily done if we decide to drop them.

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