Fix empty borrow location warnings

[ealmloff]

Rc generational boxes were using the box borrow info instead of the resolved borrow info which causes the borrow list to be empty.

This PR fixes that issue by resolving the pointer first then getting the borrow guard. It fixes the error message for this example:

use dioxus::prelude::*;

fn main() {
    dioxus::launch(app);
}

fn app() -> Element {
    let mut count = use_signal(|| 0);
    let foo = count.write_unchecked();
    let bar = count.read_unchecked();

    rsx! {
        h1 { "High-Five counter: {count}" }
        button { onclick: move |_| count += 1, "Up high!" }
        button { onclick: move |_| count -= 1, "Down low!" }
    }
}