[Flang][Driver][OpenMP] Enable options for selecting offloading phases in flang

This patch unlocks the "--offload-device-only", "--offload-host-only" and
"--offload-host-device" options available in Clang for use by the Flang driver.
These can be used to modify the behavior of the driver to select which
compilation invocations are triggered during OpenMP offloading.

Differential Revision: https://reviews.llvm.org/D147941