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

[Erich Keane via cfe-commits]

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@@ -1557,6 +1557,23 @@ def HIPManaged : InheritableAttr {
   let Documentation = [HIPManagedAttrDocs];
 }
 
+def CUDAClusterDims : InheritableAttr {
+  let Spellings = [GNU<"cluster_dims">, Declspec<"__cluster_dims__">];
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erichkeane wrote:

So the spellings should STILL not use underscores before/after.  They'll get it 'automatically' with our system (same with Declspec IIRC).  Having the underscores in the spellings before we modify it is likely to cause something odd (though I'm not sure what yet) with our attempts to add them as alternate spellings

(that is, we actually strip these before checking whether an attribute spelling matches, as we support ALL spellings like that).

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