Introduce -dsuppress-stg-free-vars flag

This breaks out control over STG free variable list output from
-dppr-debug into its own distinct flag. This makes it more discoverable
and easier to change independently from other dump output.

Test Plan: Validate

Reviewers: austin

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4140