Impove Lint check for empty cases

The Lint check for empty case alternative looks wonky to me

     ; case tyConAppTyCon_maybe (idType var) of
         Just tycon
              | debugIsOn
              , isAlgTyCon tycon
              , not (isAbstractTyCon tycon)
              , null (tyConDataCons tycon)
              , not (exprIsDeadEnd scrut)
              -> pprTrace "Lint warning: case binder's type has no constructors" (ppr var <+> ppr (idType var))
                        -- This can legitimately happen for type families
                      $ return ()
         _otherwise -> return ()

For example

   data T a = T1 !(F a) | T2 Int
   data DataConCantHappen -- No constructors
   type instance F Bool = DataConCantHappen

   f (x::T Bool) = case x of
                     T1 v -> case (v |> co) of {}
                     T2 x -> blah

   where co :: F Bool ~ Void

Here the T1 alternative in f's RHS is in fact inaccessible: T1 is a strict constructor and there are no inhabitants of (F Bool), which is equal to DataConCantHappen. GHC itself uses this a lot in HsSyn to make inactive constructors inaccessible.

So there is no hope of being sure that the scrutinee is bottoming -- except via its type. And in this test we've already checked for empty data cons.

I think we need something more like

     ; checkL (not (null alts) || exprIsDeadEnd scrut) $
       hang (text "Illegal empty case")
          2 (ppr case_bndr <+> ppr case_bndr_ty $$ ppr e)

where exprIsDeadEnd checks for empty types (which it already does, more or less).