Auto merge of #37369 - estebank:multiline-span, r=nikomatsakis

Show multiline spans in full if short enough

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 file2.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
  --> file2.rs:13:9
   |
13 |    foo(1 + bar(x,
   |        ^ trait `{integer}: std::ops::Add<()>` not satisfied
   |
```

show

```
% ./rustc file2.rs
error[E0277]: the trait bound `{integer}: std::ops::Add<()>` is not satisfied
  --> file2.rs:13:9
   |
13 |      foo(1 + bar(x,
   |  ________^ starting here...
14 | |            y),
   | |_____________^ ...ending here: trait `{integer}: std::ops::Add<()>` not satisfied
   |
```

The [proposal in internals](https://internals.rust-lang.org/t/proposal-for-multiline-span-comments/4242/6) outlines the reasoning behind this.

src/libsyntax/lib.rs

@@ -145,4 +145,7 @@ pub mod ext {
     }
 }
 
+#[cfg(test)]
+mod test_snippet;
+
 // __build_diagnostic_array! { libsyntax, DIAGNOSTICS }