`\` escape error in CR LF line mode.

[liigo]

fn main() {
    println!("Hello\
World.");
}

If I save the source to a file with CR-LF line mode (lines end with \r\n), will got the compile error:

rustc hello.rs
hello.rs:2:21: 2:22 error: unknown string escape: \r
hello.rs:2     println!("Hello\
                               ^

[huonw]

Dupe of #11669.