Box::into_unique: do the reborrow-to-raw *after* destroying the Box
Currently we first "reborrow" the box to a raw pointer, and then `forget` it. When tracking raw pointers more strictly (something I am experimenting with locally in Miri), the "use" induced by passing the box to `forget` invalidates the previously created raw pointer.
So adjust my hack from https://github.com/rust-lang/rust/pull/58429 to reorder the two operations.