Rollup merge of #56012 - RalfJung:unsafe-cell, r=nikomatsakis

avoid shared ref in UnsafeCell::get

Avoid taking a shared reference in `UnsafeCell::get`. This *should* be taking a raw reference (see https://github.com/rust-lang/rfcs/pull/2582), but that operation is not currently available, so I propose we exploit `repr(transparent)` instead and cast the pointer around.

This is required to make `UnsafeCell::get` pass the [stacked borrows implementation](https://www.ralfj.de/blog/2018/11/16/stacked-borrows-implementation.html) in miri (currently, `UnsafeCell::get` is on a whitelist, but that is of course not very satisfying). It shouldn't affect normal execution/codegen. Would be great if we could get this landed and shrink miri's whitelist!

Cc @nikomatsakis