Added clang-cl & clang-dxc logic

Additional changes to improve correctness, including preprocessing to
filter automatically added driver options, changed FileCheck strings,
etc.

The script generates two test files, since the flang tests are under the
flang directory.
    Driver/unsupported_in_drivermode.c
    Driver/flang/unsupported_in_flang.f90

Notably, As per flang.f90, "-fc1 is invoked when in
--driver-mode=flang", so tests were not generated for visibility
FlangOption; only for FC1Option.

Author: GeorgeKA