Auto merge of #46749 - SimonSapin:exorcism, r=nikomatsakis

Move PhantomData<T> from Shared<T> to users of both Shared and #[may_dangle]

After discussing https://github.com/rust-lang/rust/issues/27730#issuecomment-316432083 today with @pnkfelix and @Gankro, we concluded that it’s ok for drop checking not to be much smarter than the current `#[may_dangle]` design which requires an explicit unsafe opt-in.