Summary:
Previously --show-options showed all options that GHC accepts.
Now, it'll only show the options that have effect in non-interactive
modes.
This change also adds support for using --interactive together with
--show-options, making it show all options that have effect in the interactive
mode.
The CmdLineParser is updated to know about ghc modes, and then each flag
is annotated with which mode it has effect.
This fixes #9259.

Test Plan:
Try out --show-options with --interactive on the command line. With and without
--interactive should give different results.

Reviewers: austin

Reviewed By: austin

Subscribers: jstolarek, thomie, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D337

GHC Trac Issues: #9259

Conflicts:
	compiler/main/DynFlags.hs