Auto merge of #152737 - scottmcm:box_new_uninit_layout, r=RalfJung

Just pass `Layout` directly to `box_new_uninit`

*[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/152737)*

We have a constant for it already (used in `RawVec` for basically the same polymorphization) so let's use it.

This is a simple follow-up to rust-lang/rust#148190 from one of the comments.

Author: bors