Suggest language extension for unlifted data family

When defining an unlifted datatype or newtype, GHC helpfully tells me `Suggested fix: Perhaps you intended to use UnliftedNewtypes` or `UnliftedDatatypes`. But this does not happen with data families, as can be tested by comparing the errors for `A`, `B`, `C`, `D`:

```haskell
{-# LANGUAGE TypeFamilies, MagicHash, DataKinds #-}
module NT where

import GHC.Exts

{-
newtype A = MkA Int#

type B :: UnliftedType
data B = MkB
-}

data family C :: TYPE IntRep
newtype instance C = MkC Int#

data family D :: UnliftedType
data instance D = MkD
```