[VS Code] [Cohosting] Override completion doesn't work

[davidwengier]

We don't currently advertise for resolve, which might be a problem, but even if we do, things don't work. The current completion code we have happily re-writes the Data property of the completion item it receives, and also the one(s) it sends back. In VS this is fine as there is a serialization boundary between the LSP handler and the OOP service, but in VS Code there is no such boundary, so Roslyn ends up with cached result ids that are from our cache, and don't match etc.