Re-write diagnostics handler

[Aatch]

The current one uses a frankly obscene amount of @-allocation. It needs a major overhaul in order to make it possible to send it between tasks.

[huonw]

Triage visit: still horrible. I've made a little attempt at this, but I quickly hit various road-blocks (particularly the use of @-closures), so I'm unlikely to continue in the near future.

[steveklabnik]

I am guessing that this is src/libsyntax/diagnostics/*? It now only uses one Gc<T>.

[huonw]

@steveklabnik confusingly, it's src/libsyntax/diagnostic.rs; it now doesn't have any @ closures (since they're gone) and doesn't actually use Gc<T> at all either.

[brson]

Dupe of #10492