Fix handling of unbound constructor names in TH #14627

Authored by mgsloan on Jul 12 2018, 9:07 AM.

Description

Fix handling of unbound constructor names in TH Trac #14627

Also adds a comment to UnboundVarE clarifying that it also is used for
unbound constructor identifiers, since that isn't very clear from the
name.

Test Plan: testsuite/tests/th/T14627.hs

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4923