Let the ApronCPA use the static Method from the VariableClassification to create the name of a function return variable. As the VariableTrackingPrecision relies on the VariableClassification, all classes that are using the VariableTrackingPrecision should do so, too.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14263 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c