Summary:
Before:

    [1 of 1] Compiling Main             ( Main.hs, Main.o )
    You are using a new version of LLVM that hasn't been tested yet!
    We will try though...

After:

    [1 of 1] Compiling Main             ( Main.hs, Main.o )
    You are using an unsupported version of LLVM!
    Currently only 3.7 is supported.
    We will try though...

Before:

    [1 of 1] Compiling Main             ( Main.hs, Main.o )

    <no location info>:
        Warning: Couldn't figure out LLVM version!
                 Make sure you have installed LLVM
    ghc: could not execute: opt

After:

    [1 of 1] Compiling Main             ( Main.hs, Main.o )

    <no location info>: error:
        Warning: Couldn't figure out LLVM version!
                 Make sure you have installed LLVM 3.7
    ghc-stage1: could not execute: opt

Reviewers: austin, rwbarton, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1658