Improve tmp iterator variable name for convert_for_to_while_let

[A4-Tacks]

Example

fn main() {
    let x = vec![1, 2, 3];
    for$0 v in x {
        let y = v * 2;
    };
}

Before this PR

fn main() {
    let x = vec![1, 2, 3];
    let mut tmp = x.into_iter();
    while let Some(v) = tmp.next() {
        let y = v * 2;
    };
}

After this PR

fn main() {
    let x = vec![1, 2, 3];
    let mut iter = x.into_iter();
    while let Some(v) = iter.next() {
        let y = v * 2;
    };
}