Truncate box deref in move closures

[arora-aman]

#![feature(capture_disjoint_fields)]
#![feature(rustc_attrs)]

struct S(i32, String);

fn main() {
    let x = Box::new(S(0, String::new()));
    
    let c = move || {
        println!("{}, x.1); // currently capture x.1 by Move
    };
}

This should capture x by move.