librustc: Use the correct categorized mutable type for the pattern in
`for` loop heads.

This breaks code like:

    let x = Some(box 1i);
    for &a in x.iter() {
    }

Change this code to obey the borrow checking rules. For example:

    let x = Some(box 1i);
    for &ref a in x.iter() {
    }

Closes #16205.

[breaking-change]