Ryan Scott authored Sep 30, 2019 and Marge Bot committed Oct 08, 2019

Currently, newtype constructors are not marked as used when they are
accessed under the hood by uses of `coerce`, as described in #10347.
This fixes #10347 by co-opting the `tcg_keep` field of `TcGblEnv`
to track uses of newtype constructors in the `Coercible` solver.
See `Note [Tracking unused binding and imports]` in `TcRnTypes`.

Since #10347 is fixed, I was able to simplify the code in `TcDeriv`
slightly, as the hack described in
`Note [Newtype deriving and unused constructors]`
is no longer necessary.