Use a GDM to record the returned expression in the state when VisitReturnStmt.

Use this information to find the returned value and bind it to CallExpr in
ProcessCallExit.
And there is no need to remove dead bindings in ProcessCallExit, because
a. it would clean up the return value bound to CallExpr
b. we still would do it in the next ProcessStmt(), where we would not misclean
   up the return value.

llvm-svn: 97225