This PR completes implementation of insertion of OpGenericCastToPtr
using builtin functions started by
https://github.com/llvm/llvm-project/pull/95055 by:
* fixing errors in Tablegen definition,
* adding type inference info for `__spirv_GenericCastToPtrExplicit` kind
of wrappers, and
* hardening the test case to check correct translation of
`__spirv_GenericCastToPtrExplicit` kind of wrappers.
This PR implements insertion of OpGenericCastToPtr using builtin
functions (both opencl `to_global|local|private` and `__spirv_`
wrappers), and improves type inference.
