Switch to LLVM version 3.9
Switch to LLVM version 3.9

Test Plan:
Build and test on x86_64/linux (perf-llvm) and armhf/linux.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: RyanGlScott, thomie

Differential Revision: https://phabricator.haskell.org/D2719

olsner/ghc,olsner/ghc,ezyang/ghc,mettekou/ghc,olsner/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,mettekou/ghc,mettekou/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,olsner/ghc