Mark newtype constructors as used in the Coercible solver (#10347)

Currently, newtype constructors are not marked as used when they are accessed under the hood by uses of coerce, as described in #10347 (closed). This fixes #10347 (closed) 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 (closed) 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.