Make spans in MIR output into links

Pressing the MIR button produces output that contains comments like this:

```
// scope 0 at src/main.rs:3:2: 3:2
```

It would be nice to be able to click on this comment to jump to the corresponding row/column in the source editor, as is already possible for spans in compiler errors.  (See also rust-lang/rust-playpen#284.)