Adapt CFA nodes of Java programs to changes in revisions 14483 - 14485.

Provide easier access to function return variables through JMethodEntryNode.
Create return variable immediately when changing scope.



git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14919 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c