We weren't expanding type synonyms when determining the
injective type variables of a type, leading to certain non-injective
families being mistakenly labeled as injective (#12430). Easily fixed
with a tactical use of `coreView`.

Test Plan: make test TEST=T12430

Reviewers: bgamari, goldfire

Reviewed By: goldfire

Subscribers: goldfire, rwbarton, carter

GHC Trac Issues: #12430

Differential Revision: https://phabricator.haskell.org/D5228