[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
+to set the dimensions of a thread block cluster. This allows to group multiple thread blocks into
+a larger unit called a "cluster". `__cluster_dims__` defines the cluster size as ``(X, Y, Z)``,
+where each value is the number of thread blocks in that dimension.
+  }];
+}
+
+def CUDANoClusterAttrDoc : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+In CUDA/HIP programming, the ``__no_cluster__`` attribute can be applied to a kernel function to
+indicate that the thread block cluster feature will not be enabled at both compile time and kernel
----------------
erichkeane wrote:

Perhaps we should :)  The point of the docs here is to help newbies have SOME level of idea what the thing does so they can research it more.  Right now it is an expert-only documentation.

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