[SYCL] Issue a deprecation warning for <CL/sycl.hpp> (#13569)

Author: aelovikov-intel

sycl/include/CL/sycl.hpp

@@ -8,6 +8,10 @@
 
 #pragma once
 
+#if defined(__clang__)
+#warning "CL/sycl.hpp is deprecated, use sycl/sycl.hpp"
+#endif
+
 #include <sycl/sycl.hpp>
 
 namespace cl {

sycl/test/warnings/sycl_2020_deprecations.cpp

@@ -1,5 +1,6 @@
 // RUN: %clangxx %fsycl-host-only -fsyntax-only -ferror-limit=0 -sycl-std=2020 -Xclang -verify -Xclang -verify-ignore-unexpected=note %s
 
+// expected-warning@sycl/CL/sycl.hpp:12 {{CL/sycl.hpp is deprecated, use sycl/sycl.hpp}}
 #include <CL/sycl.hpp>
 #include <sycl/ext/intel/experimental/online_compiler.hpp>