Resync `Rc` fork from upstream `std`

[lopopolo]

Upstream PRs to library/alloc/src/rc.rs since cactusref forked from rust-lang/rust@f586d79:

• Implement RefUnwindSafe for Rc<T> rust-lang/rust#87467
• Add comments regarding superfluous !Sync impls rust-lang/rust#87530
• Fix typo “a Rc” → “an Rc” (and a few more) rust-lang/rust#88226
• Mark Arc::from_inner / Rc::from_inner as unsafe rust-lang/rust#89741
• Add #[must_use] to Rc::downgrade rust-lang/rust#89833
• Stabilize arc_new_cyclic rust-lang/rust#90666
• impl Arc::unwrap_or_clone rust-lang/rust#91589
• Improve Arc and Rc documentation rust-lang/rust#93109
• Make Weak::new const rust-lang/rust#94991
• Strict Provenance MVP rust-lang/rust#95241