Allow opt+llc from LLVM5

Summary:
This bumps our LLVM version requirement to LLVM5,
which will likely be released before GHC 8.4.

Test Plan: validate

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D3797