NH-3624

fix for https://nhibernate.jira.com/browse/NH-3624
I don't quite understand why we even have the ASTNode class instead of using antlr's CommonTree class. But I hesitate to change the meaning of NextSibling setter like this. HqlSqlWalker.LookupProperty() seems to use the NextSibling setter with the express purpose of removing a node.

Perhaps all users of the NextSibling setter should be rewritten to use other methods, for better clarity.