Fails to format doc comments on statements

[dtolnay]

Input:

fn main() {
    /// wat
    let _ = ();
}

cargo expand produces:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate std;
fn main() {
    # [ doc = " wat" ] let _ = ( ) ;
}

It seems rustfmt's normalize_doc_attributes config fails on statements (rust-lang/rustfmt#3981). For now we should just throw away doc attributes on statements before invoking rustfmt.