Ben Gamari authored Mar 15, 2017 and Ben Gamari committed Mar 15, 2017

This factors out the repetition of (log_action dflags dflags) and will
hopefully allow us to someday better abstract log output.

Test Plan: Validate

Reviewers: austin, hvr, goldfire

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3334