[flang][driver] Fix opt-level option parsing

This update makes sure that `flang-new -O2 -O0` will run at `-O0` rather
than `-O2`.

This bug was identified and originally fixed by Vyacheslav Zakharin in
https://reviews.llvm.org/D130035. I've extracted the fix into a separate
patch.

co-authored by: Vyacheslav Zakharin <vyacheslav.p.zakharin@intel.com>

Differential Revision: https://reviews.llvm.org/D130104