Add a missing update of sc_hole_ty (#16312)
(Submitted on behalf of @simonpj per his request in https://gitlab.haskell.org/ghc/ghc/issues/16312#note_201935.)
In simplCast I totally failed to keep the sc_hole_ty field of ApplyToTy (see Note [The hole type in ApplyToTy]) up to date. When a cast goes by, of course the hole type changes.
Amazingly this has not bitten us before, but #16312 (closed) finally triggered it. Fortunately the fix is simple.
Fixes #16312 (closed).