Add refinement variables for compound assignments

[smillst]

Refinement variables are not created for compound assignments. (This includes string concatenation compound assignments which are handled separately in dataflow.)

This will include changes to the following methods:
checkers.inference.dataflow.InferenceTransfer#visitAssignment
checkers.inference.dataflow.InferenceTransfer#visitStringConcatenateAssignment
checkers.inference.InferenceTreeAnnotator#visitCompoundAssignment

[smillst]

I could not reproduce this for the OSTrusted type system. I think this is because of special handling of binary trees in that type system.