Importing external memory via named NT handle reference along with pointer reference

[nikhiljnv]

cl_khr_external_memory allows importing external memory specified via NT handle through CL_EXTERNAL_MEMORY_HANDLE_OPAQUE_WIN32_KHR The spec says "CL_EXTERNAL_MEMORY_HANDLE_OPAQUE_WIN32_KHR specifies an NT handle ..." This is mostly assuming reference to external memory is specified via void* NT handle pointer. However, NT handle references can also be specified via named string.

This issue tracks discussion wrt supporting named NT handle references.