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

[Tom Honermann via cfe-commits]

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@@ -12851,6 +12851,10 @@ def err_sycl_special_type_num_init_method : Error<
   "types with 'sycl_special_class' attribute must have one and only one '__init' "
   "method defined">;
 
+//SYCL external attribute diagnostics
----------------
tahonermann wrote:

```suggestion
// SYCL external attribute diagnostics
```

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