Mispositioned help span indicator when emojis are involved

[est31]

While issue #8706 is fixed now in most places, I have found a case where the error indicator is still mispositioned:

    let b = "hello";
    println!("🦀🦀🦀🦀🦀"); let _a = b + ", World!";

gives a mispositioned span:

  |
3 |     println!("🦀🦀🦀🦀🦀"); let _a = b.to_owned() + ", World!";
  |                                 ^^^^^^^^^^^^

on rust stable (1.23.0) as well as beta (1.24.0-beta.2) and nightly (73ac5d6 2018-01-11).

It should underline the b.to_owned().

I believe that this issue has the same root cause as #47377.

[est31]

#47407 fixed this