Previously, derived `Generic1` instances could have associated `Rep1`
type family instances with unbound variables, such as in the following
example:

```lang=haskell
data T a = MkT (FakeOut a) deriving Generic1
type FakeOut a = Int

==>

instance Generic1 T where
  type Rep1 T = ... (Rec0 (FakeOut a))
```

Yikes! To avoid this, we simply map the last type variable in a
derived `Generic1` instance to `Any`.

Test Plan: make test TEST=T15012

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: simonpj, thomie, carter

GHC Trac Issues: #15012

Differential Revision: https://phabricator.haskell.org/D4602