swift234

Make the linking pass add new functions to the call graph.

If we already have a call graph built, we should add any newly
deserialized functions to it. This is happening in other places we're
deserializing, but wasn't happening in the linking pass, and we've been
getting away with it because we invalidate the entire call graph and
rebuild it when we next need it.

An upcoming change will invalidate only the call graph node for the
current function, so we really need to add these new functions to the
call graph as we deserialize them.