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

[Tom Honermann via cfe-commits]

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@@ -7206,6 +7206,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_EnumExtensibility:
     handleEnumExtensibilityAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_SYCLExternal:
+    S.SYCL().handleExternalAttr(D, AL);
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tahonermann wrote:

I think `handleSimpleAttribute()` can be called here and the diagnostics handled elsewhere. See additional comments in `SemaSYCL::handleExternalAttr()`.

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