[clang][SYCL] Allow structs as free function kernel arguments (#15334)

Support for non-decomposed structs with pointers is added.
Doesn't include support for structs containing SYCL special types aka
accessors, streams and etc.

Author: Fznamznon

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12433,6 +12433,12 @@ def err_sycl_kernel_incorrectly_named : Error<
   "'-fsycl-unnamed-lambda' to enable unnamed kernel lambdas"
   "}0">;
 
+// SYCL free function kernels extension.
+def note_free_function_kernel_param_type_not_fwd_declarable : Note<
+  "%0 is not forward declarable">;
+def note_free_function_kernel_param_type_not_supported : Note<
+  "%0 is not yet supported as a free function kernel parameter">;
+
 def err_sycl_kernel_not_function_object
     : Error<"kernel parameter must be a lambda or function object">;
 def err_sycl_restrict : Error<