Summary:
Unboxed tuples have `RuntimeRep` arguments which `-XDeriveFunctor` was
mistaking for actual data constructor arguments. As a result, a derived
`Functor` instance for a datatype that contained an unboxed tuple would
generate twice as many arguments as it needed for an unboxed tuple pattern
match or expression. The solution is to simply put `dropRuntimeRepArgs` in the
right place.

Fixes #12399.

Test Plan: ./validate

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie, osa1

Differential Revision: https://phabricator.haskell.org/D2404

GHC Trac Issues: #12399