Emit references relationship for method overrides

[varungandhi-src]

In scip-java, if you override a method, the override will get a new symbol and an overrides relationship that connects it to the override symbol. We don't emit that relationship right now, so we should fix that. Otherwise, find references on overriden methods won't work correctly.

[varungandhi-src]

This should be simpler to implement after #116 lands, because that patch adds support for tracking relationships.