Add ‘help’ messages to rustc

This adds ‘help’ diagnostic messages to rustc. This is used for anything that provides help to the user, particularly the `--explain` messages that were previously integrated into the relevant error message.

They look like this:

```
match.rs:10:13: 10:14 error: unreachable pattern [E0001]
match.rs:10             1 => {},
                        ^
match.rs:3:1: 3:38 note: in expansion of foo!
match.rs:7:5: 20:2 note: expansion site
match.rs:10:13: 10:14 help: pass `--explain E0001` to see a detailed explanation
```

(`help` is coloured cyan.) Adding these errors on a separate line stops the lines from being too long, as discussed in #16619.