Add a missing update of sc_hole_ty (Trac #16312)
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 Trac #16312 finally
triggered it. Fortunately the fix is simple.
Fixes Trac #16312.
(cherry picked from commit ef36b10233e46f8462f42531e8e0c1ebe0bcb1eb)