When dealing with multiline spans that span few lines, show the complete
span instead of restricting to the first character of the first line.

For example, instead of:

```
% ./rustc foo.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
  --> foo.rs:13:9
   |
13 |    foo(1 + bar(x,
   |        ^ trait `{integer}: std::ops::Add<()>` not satisfied
   |
```

show

```
% ./rustc foo.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
  --> foo.rs:13:9
   |
13 |      foo(1 + bar(x,
   |  ________^ starting here...
14 | |            y),
   | |_____________^ ...ending here: trait `{integer}: std::ops::Add<()>` not satisfied
   |
```