Rollup merge of rust-lang#148079 - taj-p:master, r=Mark-Simulacrum

Rename `downcast_[ref|mut]_unchecked` -> `downcast_unchecked_[ref|mut]`

## Intent

Renames `downcast_[ref|mut]_unchecked` to `downcast_unchecked_[ref|mut]` because we want to emphasise that it is the downcast that is unsafe, not the aliasing per:

rust-lang#90850 (comment)

## Tracking Issue: rust-lang#90850 (comment)

cc `@marc0246`

Author: Zalathar