Clang: move gnome-builder-clang to D-Bus
Using JSONRPC-GLib restricts a bunch of neat things we could do if we used D-Bus for the serialization protocol from UI to worker process. For example, we could use memfd for large completion result sets and pass that FD to the UI process to avoid building large messages.
We can also have more consistent command cancellations than we have now.
We already dynamically upgrade to GVariant from JSONRPC, so the performance isn't of the upmost issue. But it would be nice to be able to use actual interfaces rather than brittle conditionals like we do now.