[flang] Fix Driver/do_concurrent_to_omp_cli.f90 test not to use runtime (#135485)
Fix Flang invocation in `Driver/do_concurrent_to_omp_cli.f90` test to
run compilation step only, to fix testing when building with
`-DFLANG_INCLUDE_RUNTIME=OFF`. The test is only concerned with warning
being emitted by the compiler, so there is no need to link the resulting
executable.