Move eta-reduced coaxiom compatibility handling quirks into FamInstEnv.

Authored by mniip on Nov 1 2018, 5:33 PM.

Description

Move eta-reduced coaxiom compatibility handling quirks into FamInstEnv.

The quirk caused an issue where GHC concluded that 'D' is possibly
unifiable with 'D a' (the two types could have the same kind if D is a
data family).

Test Plan:
Ensure T9371 stays fixed.
Introduce T15704

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: RyanGlScott, rwbarton, carter

GHC Trac Issues: Trac #15704

Differential Revision: https://phabricator.haskell.org/D5206