Rollup merge of #111654 - JoJoJet:unsafe-cell-from-mut-lib, r=joshtriplett

Add a conversion from `&mut T` to `&mut UnsafeCell<T>`

Provides a safe way of downgrading an exclusive reference into an alias-able `&UnsafeCell<T>` reference.

ACP: rust-lang/libs-team#198.

Author: Dylan-DPC