[Flang] Add debug flag to enable current debug information pass

While a pass exists to generate basic debug information, currently there is not a corresponding flag to enable it.
This patch adds support for activating this pass at any debug level >= -g1, as well as emiting a warning for higher levels that the functionality is not yet fully implemented.

This patch also adds -g and -gline-tables-only to appear when `flang-new` --help is run

Depends on D142347.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D146814