Default kind vars in tyfams with -XNoPolyKinds

  We should still default kind variables in type families
  in the presence of -XNoPolyKinds, to avoid suggesting enabling
  -XPolyKinds just because the function arrow introduced kind variables,
  e.g.

    type family F (t :: Type) :: Type where
      F (a -> b) = b

  With -XNoPolyKinds, we should still default `r :: RuntimeRep`
  in `a :: TYPE r`.

  Fixes #20584