Allow type family in data family return kind

GHC currently allows

data family DF1 :: k1 -> k2

where it's expected (and checked) that all data instances have a return kind of Type. (Perhaps k2 expands to Type -> Type, for example.)

However, it rejects

type family TF (x :: Type) :: Type
data family DF2 :: x -> TF x

when that's clearly just as sensible as the first definition.

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