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

[Tom Honermann via cfe-commits]

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@@ -12908,6 +12908,14 @@ 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">;
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tahonermann wrote:

Searching for "can only" in `DiagnosticSemaKinds.td` doesn't suggest a consistent formulation. There are quite a few examples that match the currently proposed spelling.

If "attribute" is added after `sycl_external`, then I would suggest adding "the" before it; "the 'sycl_external' attribute ...". 

We probably should use `'clang::sycl_external'` for consistency with diagnostics that use substitution to provide the attribute name; see the "... attribute ignored" diagnostics for example.

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