[SYCL] Include backend-specific header if exists (#4783)

https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:headers-and-namespaces
introduces the backend specific headers "sycl/backend/<backend_name>.hpp"
CTS tests assume that if backend macro is defined (e.g. SYCL_BACKEND_OPENCL)
it is fully functional without extra includes. So the patch adds
include of the backend specific header when the backend is enabled.

The change should fix the recent massive failure of CTS tests with the error:
implicit instantiation of undefined template 'sycl::interop<sycl::backend::opencl, sycl::platform>'
Co-authored-by: Mikhail Lychkov <mikhail.lychkov@intel.com>
Co-authored-by: Mikhail Lychkov <mikhail.lychkov@intel.com>

Author: vladimirlaz

sycl/include/CL/sycl/feature_test.hpp.in

@@ -11,6 +11,7 @@ __SYCL_INLINE_NAMESPACE(cl) {
 namespace sycl {
 
 #define SYCL_BACKEND_OPENCL 1
+
 // Feature test macro definitions
 
 // TODO: Move these feature-test macros to compiler driver.
@@ -46,3 +47,12 @@ namespace sycl {
 
 } // namespace sycl
 } // __SYCL_INLINE_NAMESPACE(cl)
+
+#include <CL/sycl/backend/opencl.hpp>
+// Level Zero backend header depends on external headers and should be
+// included by user application directly
+// #include <sycl/ext/oneapi/backend/level_zero.hpp>
+
+#if SYCL_EXT_ONEAPI_BACKEND_CUDA
+#include <CL/sycl/backend/cuda.hpp>
+#endif