For some time now, type-level operators such as '+' have been treated as
type constructors, rahter than type variables.  This pathc fixes TH's
`lookupName` function to account for this behavior.

Reviewers: bgamari, austin, goldfire, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: Phyx, thomie

Differential Revision: https://phabricator.haskell.org/D3025

GHC Trac Issues: #11046