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

[Erich Keane via cfe-commits]

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@@ -1572,6 +1572,23 @@ def HIPManaged : InheritableAttr {
   let Documentation = [HIPManagedAttrDocs];
 }
 
+def CUDAClusterDims : InheritableAttr {
+  let Spellings = [GNU<"cluster_dims">, Declspec<"cluster_dims">];
+  let Args = [ExprArgument<"X">, ExprArgument<"Y", 1>, ExprArgument<"Z", 1>];
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erichkeane wrote:

```suggestion
  let Args = [ExprArgument<"X">, ExprArgument<"Y",  /*opt=*/1>, ExprArgument<"Z", /*opt=*/1>];
```
We're SUPPOSED to have these, though I suspect we're really lazy/inconsistent about these.

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