[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
+launch time. Note: this is a LLVM/Clang only attribute.
----------------
erichkeane wrote:

Instead of as a note, I'd suggest making the first sentence be: `..., the Clang/LLVM-exclusive __no_cluster__ attribute`

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