Add ghc-flag directory for -XPatternGuards
Add ghc-flag directory for -XPatternGuards

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1987

(cherry picked from commit e764ede35f5c5b2c41e1670c6a9b831e0a70cd17)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc