[flang][driver] Add support for the "-init-only" option

Adding the `-init-only` option and corresponding frontend action to
generate a diagnostic.

`-init-only` vs `-test-io`:
`-init-only` ignores the input (it never calls the prescanner)
`-test-io` is similar to `-init-only`, but does read and print the input
without calling the prescanner.

This patch also adds a Driver test to check this action.

Reviewed By: awarzynski, AMDChirag

Differential Revision: https://reviews.llvm.org/D102849