Auto merge of #149079 - zachs18:clone_from_ref, r=Mark-Simulacrum

Add `Box::clone_from_ref` and similar under `feature(clone_from_ref)`

Tracking issue: rust-lang/rust#149075

Accepted ACP: rust-lang/libs-team#483

This PR implements `clone_from_ref` (and `try_*` and `_*in` variants), to get a `Box<T>`, `Arc<T>`, or `Rc<T>` by cloning from a `&T` where `T: CloneToUninit`.

The "Implement..." commits replace some ad-hoc conversions with `clone_from_ref` variants, which can be split out to a separate PR if desired.

This PR will conflict with rust-lang/rust#148769 due to usage of `Layout::dangling` (which that PR is renaming to `dangling_ptr`), so they should not be rolled up together, and the one which merges later will need to be amended.

Author: bors