Auto merge of #145608 - Darksonn:derefmut-pin-fix, r=lcnr

Prevent downstream `impl DerefMut for Pin<LocalType>`

The safety requirements for [`PinCoerceUnsized`](https://doc.rust-lang.org/stable/std/pin/trait.PinCoerceUnsized.html) are essentially that the type does not have a malicious `Deref` or `DerefMut` impl. However, the `Pin` type is fundamental, so the end-user can provide their own implementation of `DerefMut` for `Pin<&SomeLocalType>`, so it's possible for `Pin` to have a malicious `DerefMut` impl. This unsoundness is known as rust-lang/rust#85099.

Unfortunately, this means that the implementation of `PinCoerceUnsized` for `Pin` is currently unsound. To fix that, modify the impl so that it becomes impossible for downstream crates to provide their own implementation of `DerefMut` for `Pin` by abusing a hidden struct that is not fundamental.

This PR is a breaking change, but it fixes rust-lang/rust#85099. The PR supersedes rust-lang/rust#144896.

r? lcnr

Author: bors