[OpenMP] Add options to only compile the host or device when offloading

OpenMP recently moved to the new offloading driver, this had the effect
of making it more difficult to inspect intermediate code for the device.
This patch adds `-foffload-host-only` and `-foffload-device-only` to
control which sides get compiled. This will allow users to more easily
inspect output without needing the temp files.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D124220