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

[Tom Honermann via cfe-commits]

================
@@ -12856,6 +12856,14 @@ 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
+def err_sycl_attribute_invalid_linkage : Error<
+  "'sycl_external' can only be applied to functions with external linkage">;
+def err_sycl_attribute_avoid_main : Error<
+  "'sycl_external' cannot be applied to main function">;
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tahonermann wrote:

For consistency with other diagnostics (see `err_coroutine_invalid_func_context`).
```suggestion
def err_sycl_attribute_avoid_main : Error<
  "'sycl_external' cannot be applied to the 'main' function">;
```

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