Adjust reporting of unused tyvars in data FamInsts

This commit adjusts the validity checking of data family
instances to improve the reporting of unused type variables.

See Note [Out of scope tvs in data family instances] in GHC.Tc.Validity.

The problem was that, in a situation such as

  data family D :: Type
  data instance forall (d :: Type). D = MkD

the RHS passed to 'checkFamPatBinders' would be the TyCon app

  R:D d

which mentions the type variable 'd' quantified in the user-written
forall. Thus, when computing the set of unused type variables in
the RHS of the data family instance, we would find that 'd' is used,
and report a strange error message that would say that 'd' is not
bound on the LHS.

To fix this, we special-case the data-family instance case,
manually extracting all the type variables that appear in the
arguments of all the data constructores of the data family instance.

Fixes #23778