Add feature query into cl_ext_cxx_for_opencl

For OpenCL C feature test macros there is an API's `CL_DEVICE_OPENCL_C_FEATURES` device query.

C++ for OpenCL is not inherited from OpenCL 3.0 yet but it is using C++20's feature macros that is the same concept as in OpenCL 3.0 (See details in https://github.com/KhronosGroup/OpenCL-Docs/pull/582). For consistency, it might make sense to publish a new revision of the `cl_ext_cxx_for_opencl` extension adding a `CL_DEVICE_CXX_FOR_OPENCL_FEATURES` query.

Start by reviewing the cl_ext_cxx_for_opencl extension sources and the referenced OpenCL Docs PR 582, then compare the existing CL_DEVICE_OPENCL_C_FEATURES device query. Done means a new extension revision defines the CL_DEVICE_CXX_FOR_OPENCL_FEATURES query consistently with the C++20 feature-macro model.

Written by the indexing model from the issue text.