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

[Tom Honermann via cfe-commits]

================
@@ -1641,6 +1641,13 @@ def DeviceKernel : DeclOrTypeAttr {
 }];
 }
 
+def SYCLExternal : InheritableAttr {
+  let Spellings = [Clang<"sycl_external">];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let LangOpts = [SYCLDevice];
----------------
tahonermann wrote:

In restrospect, I think we should allow the `sycl_external` attribute to be present for SYCL host compilation. It won't have an affect on what is emitted on the host side, but will allow for semantic diagnostics to be issued. It might be useful for SYCL implementations like AdaptiveCpp that don't (necessarily) perform a separate device compilation.
```suggestion
  let LangOpts = [SYCLHost, SYCLDevice];
```

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