Make Atomic*::from_mut return &mut Atomic*

[WaffleLapkin]

impl Atomic* {
    pub fn from_mut(v: &mut bool) -> &mut Self;
    //                               ^^^^---- previously was just a &
}

This PR makes from_mut atomic methods tracked in #76314 return unique references to atomic types, instead of shared ones. This makes from_mut and get_mut inverses of each other, allowing to undo either of them by the other.

r? @RalfJung
(as Ralf was concerned about this)

[RalfJung]

I think this would make more sense, but it is inconsistent with Cell::from_mut.

And I don't think I can approve this. Cc @rust-lang/libs-api

[m-ou-se]

@bors r+

[bors]

📌 Commit 2b03ed1 has been approved by m-ou-se