[Clang] Change host/device only compilation to a driver mode

We use the flags `--offload-host-only` and `--offload-device-only` to
change the driver's code generation for offloading programs. These are
currently parsed out independently in many places. This patch simply
refactors this to work as a mode for the Driver. This stopped us from
emitting warnings if unused because it's always used now, but I don't
think this is a great loss.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D127515