Enforce Rust 2018 idioms

With the current diagnostics emitter of Rust, the `-Z ui-testing` flag
allows to 'anonymize' line numbers in the UI test output.

This means that a line such as:

     2 |     concat!(b'f');

is turned into:

    LL |     concat!(b'f');

This is done because it makes the diff of UI test output changes much
less noisy.

To support this with `annotate-snippet`, we add a second parameter to
`DisplayListFormatter` that, when true, replaces the line numbers in the
left column with the text `LL`. The replacement text is always `LL` and
it does not affect the initial location line number.

In the new `annotate-snippet` emitter in rustc, we can then use this
parameter depending on whether the `-Z ui-testing` flag is set or not.