This amounts to enabling PolyKinds in GHC.Generics. However, explicit
kind signatures must be applied to the datatypes and typeclasses in
GHC.Generics to ensure that the Core which TcGenGenerics generates
is properly kinded.

Several of the typeclasses in GHC.Generics could be poly-kinded, but
this differential does not attempt to address this, since D493 already
addresses this.

Test Plan: ./validate

Reviewers: hvr, austin, dreixel, bgamari

Reviewed By: austin, dreixel, bgamari

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D1166

GHC Trac Issues: #10775