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

[zachs18]

Tracking issue: #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 #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.

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.