[Flang][Driver] Add support for fopenmp-is-device and fembed-offload-object to Flang ToolChain

This allows-fembed-offload-object's and -fopenmp-is-device
compiler invocation arguments to be passed to the Flang frontend
during split compilation when offloading in OpenMP.

An example use case is when passing an offload-arch alongside
-fopenmp to embed device objects compiled for the offload-arch
within the host architecture.

This borrows from existing clangDriver+Clang.h/.cpp work and the intent
is currently to reuse as much of the existing infrastructure and design as
we can to achieve offloading for Flang+OpenMP. An overview of
Clang's offloading design can be found
here: https://clang.llvm.org/docs/OffloadingDesign.html

Reviewers:
awarzynski
jhuber6

Differential Revision: https://reviews.llvm.org/D145815