[HIPSPV] Convert HIP kernels to SPIR-V kernels

This patch translates HIP kernels to SPIR-V kernels when the HIP
compilation mode is targeting SPIR-S. This involves:

* Setting Cuda calling convention to CC_OpenCLKernel (which maps to
  SPIR_KERNEL in LLVM IR later on).

* Coercing pointer arguments with default address space (AS) qualifier
  to CrossWorkGroup AS (__global in OpenCL). HIPSPV's device code is
  ultimately SPIR-V for OpenCL execution environment (as
  starter/default) where Generic or Function (OpenCL's private) is not
  supported as storage class for kernel pointer types. This leaves the
  CrossWorkGroup to be the only reasonable choice for HIP buffers.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D109818