[SYCL] Comment why we cannot use the opencl_generic attribute

Author: Pavel Samolysov

sycl/include/CL/sycl/access/access.hpp

@@ -123,6 +123,8 @@ constexpr bool modeWritesNewData(access::mode m) {
 #define __OPENCL_LOCAL_AS__ __attribute__((opencl_local))
 #define __OPENCL_CONSTANT_AS__ __attribute__((opencl_constant))
 #define __OPENCL_PRIVATE_AS__ __attribute__((opencl_private))
+// TODO cannot be defined as __attribute__((opencl_generic)) since clang doesn't
+// support the generic address space for SYCL.
 #define __OPENCL_GENERIC_AS__
 #else
 #define __OPENCL_GLOBAL_AS__