This is pretty straightforward, it just adds a builtin to return a
pointer to a resource handle. This maps to a dx intrinsic.
The shape of this builtin and the underlying intrinsic will likely
shift a bit as this implementation becomes more feature complete, but
this is a good basis to get started.
Depends on D128569.
Differential Revision: https://reviews.llvm.org/D130016
Chris Bieneman