[flang][driver] Forward `-fopenmp`/`-fopenacc` to the host compiler

This patch only modifies `flang` - the bash wrapper script.

`-fopenmp`/`-fopenacc` are required to enable the OpenMP/OpenACC
extension in the frontend and to make sure that the required libraries
are linked when generating the final binary. This patch makes sure that
`-fopnemp`/`-fopenacc` is used for both unparsing and the code
generation (via the host compiler).

Differential Revision: https://reviews.llvm.org/D106871