rustfmt_skip is ignored on statements

[solson]

Sort of a continuation of #1346. Note that attributes on statements are a stable feature now. Basic example:

fn main() {
    #[cfg_attr(rustfmt, rustfmt_skip)]
    foo(
        1, 2, 3, 4,
        1, 2,
        1, 2, 3,
    );
}

Clicking Format on this code should make no change, but it gets reformatted, so you have to put the attribute on the whole function instead, which may contain other code you do want rustfmt to reformat.

Thanks to #1602, rustfmt_skip actually does work with an expression attribute, but these are currently on nightly only.