[Clang] Make enabling the new driver more generic

In preparation for allowing other offloading kinds to use the new driver
a new opt-in flag `-foffload-new-driver` is added. This is distinct from
the existing `-fopenmp-new-driver` because OpenMP will soon use the new
driver by default while the others should not.

Reviewed By: yaxunl, tra

Differential Revision: https://reviews.llvm.org/D123325