[SYCL] How OpenCL device mode is being reused for SYCL

This is to track the discussion about the device compilation flow in the Clang frontend.

There are multiple ways OpenCL features can be reused:
- At source level by implementing SYCL libraries using C++ for OpenCL code and enabling OpenCL in LangOpts for SYCL device.
- At AST level by mapping SYCL constructs onto OpenCL AST nodes.
- At IR CodeGen level by re-using OpenCL CodeGen functionality.

The objective is to avoid code duplication as much as possible and to keep code base simpler/easier to maintain.