[flang] Make `flang` translate `-M{fixed|free}` into `-f{fixed|free}-form`

We are only adding this in `flang` - the bash wrapper script for the
driver. We don't have any specific plans to support `-M{fixed|free}` in
`flang-new` (i.e. the actual driver).

That feature was requested by one of our users.

Differential Revision: https://reviews.llvm.org/D107081