Add undo_leak to reset RefCell borrow state

This method is complementary for the feature cell_leak added in an
earlier PR. It allows *safely* reverting the effects of leaking a borrow guard by
statically proving that such a guard could not longer exist. This was
not added to the existing `get_mut` out of concern of impacting the
complexity of the otherwise pure pointer cast and because the name
`get_mut` poorly communicates the intent of resetting remaining borrows.

This is a follow-up to #68712 and uses the same tracking issue, #69099,
as these methods deal with the same mechanism and the idea came up
[in a review comment](https://github.com/rust-lang/rust/pull/68712#discussion_r384670041).

@dtolnay who reviewed the prior PR.
cc @RalfJung