raaiq1

1) remove unused c-tor of SYCLMemObjT: Details: include/CL/sycl/detail/sycl_mem_obj_t.hpp:79 2) remove unused getBufSizeForContext accepting cl_mem: Details: include/CL/sycl/detail/sycl_mem_obj_t.hpp:299

Removed from L0 plugin: 1) make_context(const std::vector &DeviceList, pi_native_handle NativeHandle) - Details: [source/backend/level_zero.cpp:65](https://github.com/intel/llvm/blob/784cfa13046e4874e1b8173e44231987bc7db2f8/sycl/source/backend/level_zero.cpp#L64) 2) make_queue(const context &Context, pi_native_handle NativeHandle) - Details: [source/backend/level_zero.cpp:93](https://github.com/intel/llvm/blob/784cfa13046e4874e1b8173e44231987bc7db2f8/sycl/source/backend/level_zero.cpp#L101)

Added support when linking iostream_proxy.hpp against libc++ instead of libstdc++

This is to complement this [PR](https://github.com/intel/llvm/pull/6469) which removes \ headers to comply with LLVM Coding Standards. Signed-off-by: Rauf, Rana