[SYCL] Emit kernel args size warning for real sycl headers (#2554)

It turned out that real sycl headers are included as "system" headers
and the actual warning points to function defined there, so the warning
about kernel arguments size wasn't emitted.

Author: Fznamznon

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -11020,7 +11020,7 @@ def err_sycl_restrict : Error<
   "}0">;
 def warn_sycl_kernel_too_big_args : Warning<
   "size of kernel arguments (%0 bytes) may exceed the supported maximum "
-  "of %1 bytes on some devices">, InGroup<SyclStrict>;
+  "of %1 bytes on some devices">, InGroup<SyclStrict>, ShowInSystemHeader;
 def err_sycl_virtual_types : Error<
   "No class with a vtable can be used in a SYCL kernel or any code included in the kernel">;
 def note_sycl_recursive_function_declared_here: Note<"function implemented using recursion declared here">;

sycl/test/basic_tests/args-size-overflow.cpp

@@ -0,0 +1,21 @@
+// RUN: %clangxx -fsycl -fsycl-device-only -Xclang -verify -Wsycl-strict %s -fsyntax-only
+
+#include <CL/sycl.hpp>
+
+using namespace cl::sycl;
+
+// expected-warning@../../include/sycl/CL/sycl/handler.hpp:919 {{size of kernel arguments (8068 bytes) may exceed the supported maximum of 2048 bytes on some devices}}
+
+int main() {
+  struct S {
+    int A;
+    int B;
+    int Array[2015];
+  } Args;
+  queue myQueue;
+  myQueue.submit([&](handler &cgh) {
+    // expected-note@+1 {{in instantiation of function template specialization 'cl::sycl::handler::single_task}}
+    cgh.single_task<class kernel>([=]() { (void)Args; });
+  });
+  return 0;
+}