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

[Mariya Podchishchaeva via cfe-commits]

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@@ -476,6 +476,33 @@ The SYCL kernel in the previous code sample meets these expectations.
   }];
 }
 
+def SYCLExternalDocs : Documentation {
+  let Category = DocCatFunction;
+  let Heading = "sycl_external";
+  let Content = [{
+The ``sycl_external`` attribute (or the ``SYCL_EXTERNAL`` macro) can only be applied to
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Fznamznon wrote:

I am not sure it is a good idea to mention the macro unless we introduce it with this PR too.

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