WIP: Fix #15656 by extending -Wall with appropriate flags
Summary: We extend -Wall with incomplete-uni-patterns and incomplete-record-updates as per GHC proposal #12 (closed)[1].
However, doing so gives rise to warnings due to code that is considered unsafe per these two flags. For now, we just suppress these warnings to make the build less noisy. The proper solution would be to refine things so that these warnings go away. This idea is tracked by Trac #16041.
[1] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0012-Wall-uni-patterns.rst
[skip ci]
Test Plan: make test TEST=T15656
Reviewers: RyanGlScott, bgamari
Subscribers: rwbarton, carter
GHC Trac Issues: #15656 (closed)
Differential Revision: https://phabricator.haskell.org/D5415