Alec Theriault authored Dec 11, 2018 and Ben Gamari committed Dec 11, 2018

Adds a `-fenable-ide-info` flag which instructs GHC to generate `.hie`
files (see the wiki page:
https://ghc.haskell.org/trac/ghc/wiki/HIEFiles).

This is a rebased version of Zubin Duggal's (@wz1000) GHC changes for
his GSOC project, as posted here:
https://gist.github.com/wz1000/5ed4ddd0d3e96d6bc75e095cef95363d.

Test Plan: ./validate

Reviewers: bgamari, gershomb, nomeata, alanz, sjakobi

Reviewed By: alanz, sjakobi

Subscribers: alanz, hvr, sjakobi, rwbarton, wz1000, carter

Differential Revision: https://phabricator.haskell.org/D5239