Auto merge of #4972 - varkor:lazy-cell-unused-unsafe, r=alexcrichton

Allow unused_unsafe in LazyCell in preparation for lib change

https://github.com/rust-lang/rust/pull/47204 makes `UnsafeCell::into_inner` safe, which means `LazyCell::into_inner` will no longer need an `unsafe` block. `LazyCell` is a blocker for the change in Rust: this fix should allow the change to take place.