unused_mut lint improvement: warn when variable only copied into closure

[rphmeier]

Minimal example of the lint failure to fire:

fn foo() -> bool {
    let mut res = false;
    
    let mut do_thing = move || res = true;
    do_thing();
    
    res
}

foo unconditionally returns false, but the top-level binding of res doesn't get an unused_mut warning.

[jakubadamw]

This is related to or even a duplicate of #37707 and it will be probably be fixed as part of #51003.