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

[Erich Keane via cfe-commits]

================
@@ -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
----------------
erichkeane wrote:

Want to close out docs a little here.  Perhaps:

"The `cluster_dims` attribute, conventionally exposed using the `__cluster_dims__` macro...". then referring to it as its macro for the rest of the doc.  WDYT?

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