Implement `RefUnwindSafe` for `Rc<T>`
This PR implements `RefUnwindSafe` for `Rc<T>`, where `T: RefUnwindSafe`.
This impl was omitted by an apparent oversight. `Rc<T>` already implements `UnwindSafe`. `Arc<T>` implements both `UnwindSafe` and `RefUnwindSafe`. There is no reason why an `&Rc<T>` is any less unwind safe than a `Rc<T>` or an `&Arc<T>`, so this should be safe to add.
Resolves#45924.
4061c04079