[Flang][Driver] Add warning support for invalid R_Group options

With the R_Group options, invalid values e.g. '-Rpa' will not emit
a warning like clang. This patch enables warning reporting, as
well as suggestions on what option the user intended to select.

Depends on D158174 and D158436. The former, adds backend
support to R_Group options while the latter, implements
regex support with some tests refactoring that cause a merge
conflict.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D158593