Set ast node's guid properly

This pull request is to fix defect [MAGN-4659](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4659).

That's because when calculating change set for delta execution, we need to set modified ast node's GUID to the corresponding UI node's GUID. But the logic here is set *all modified ast nodes' GUID* to *the last UI node's GUID*:
```
foreach (var st in modifiedSubTrees)
{
    ...
    deltaAstList.AddRange(modifiedASTList);
    // WRONG - change previous AST nodes
    foreach (AssociativeNode node in deltaAstList)
    {
        var bnode = node as BinaryExpressionNode;
        ...
        bnode.guid = st.GUID;
        ...
   }
}
```

@junmendoza , please review the change. Thanks.