This returns true if the type ty is “trivial” for
dropck-outlives – that is, if it doesn’t require any types to
outlive. This is similar but not quite the same as the
needs_drop test in the compiler already – that is, for every
type T for which this function return true, needs-drop would
return false. But the reverse does not hold: in particular,
needs_drop returns false for PhantomData, but it is not
trivial for dropck-outlives.