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

[Erich Keane 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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erichkeane wrote:

I'd suggest spelling these differently.  Either:

`'sycl_external' attribute`
OR:
`[[sycl_external]]`.

Probably the latter if spellings list is changed above.

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