Simon Jakobi authored Jun 14, 2018 and Ben Gamari committed Jun 14, 2018

We were using Map.fromDistinctAscList to deserialize a
(Map Name HsDocString). As the Names' Uniques had changed, we
ended up with an invalid map in which we couldn't lookup certain keys.

Switching to Map.fromList fixed the issue.

Added comments in several places.

Reviewers: alexbiehl, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15240

Differential Revision: https://phabricator.haskell.org/D4816