In the VariableClassification, use the pseudo variable for return values
of functions that is defined by the ReturnStatementEdge,
instead of an own definition.
All other code should also use the same variable.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14486 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c