Auto merge of #85758 - petertodd:2021-revert-manuallydrop-clone-from, r=m-ou-se

Revert #85176 addition of `clone_from` for `ManuallyDrop`

Forwarding `clone_from` to the inner value changes the observable behavior, as previously the inner value would *not* be dropped by the default implementation.

Frankly, this is a super-niche case, so #85176 is welcome to argue the behavior should be otherwise! But if we overrride it, IMO documenting the behavior would be good.

Example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=c5d0856686fa850c1d7ee16891014efb