[clang] Add sycl_external attribute (PR #140282)

[Tom Honermann via cfe-commits]

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@@ -1545,6 +1550,18 @@ def SYCLKernel : InheritableAttr {
   let Documentation = [SYCLKernelDocs];
 }
 
+def GlobalStorageNonLocalVar : SubsetSubject<Var,
+                                             [{S->hasGlobalStorage() &&
+                                               !S->isLocalVarDeclOrParm()}],
+                                             "global variables">;
+
+def SYCLExternal : InheritableAttr {
+  let Spellings = [GNU<"sycl_external">];
+  let Subjects = SubjectList<[Function, GlobalStorageNonLocalVar]>;
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tahonermann wrote:

In SYCL 2020, the `SYCL_EXTERNAL` macro only applies to functions. Intel downstream repos allow the `sycl_device` attribute (which is used to implement `SYCL_EXTERNAL` there) to be applied to variables, but that isn't needed for SYCL 2020 conformance. If we later find a need to extend the semantics of `sycl_external` to variables, we should check with the SYCL WG to see if additions to the SYCL specification to support such semantics are warranted.

https://github.com/llvm/llvm-project/pull/140282