Vladislav Zavialov authored Jun 17, 2018 and Ben Gamari committed Jun 17, 2018

Add a flag `-Werror=compat` to GHC which has the effect of `-Werror=x
-Werror=y ...`, where `x, y, ...` are warnings from the `-Wcompat`
option group.

Test Plan: ./validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15278

Differential Revision: https://phabricator.haskell.org/D4860