[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[via cfe-commits]

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@@ -1641,6 +1641,13 @@ def DeviceKernel : DeclOrTypeAttr {
 }];
 }
 
+def SYCLExternal : InheritableAttr {
+  let Spellings = [Clang<"sycl_external">];
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schittir wrote:

Separate PR it is! 
I'll follow this spelling: `let Spellings = [CXX11<"clang", "sycl_external">];`

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