[CUDA][SPIRV] Use OpenCLKernel CC for CUDA -> SPIRV

Select the OpenCLKernel calling convention for kernels when compiling
CUDA targeting SPIR-V.

In this way the generated LLVM IR will have a spir_kernel calling
convention that will be translated to an OpEntryPoint when converting
to SPIRV.

Reviewed By: jlebar, tra

Differential Revision: https://reviews.llvm.org/D114407