[OpenMP] Make clang argument handling for the new driver more generic

In preparation for accepting other offloading kinds with the new driver,
this patch makes the way we handle offloading actions more generic. A
new field to get the associated device action's toolchain is used rather
than manually iterating a list. This makes building the arguments easier
and makes sure that we doin't rely on any implicit ordering.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D123313