Support data-modifying commands (INSERT/UPDATE/DELETE) in WITH.

This patch implements data-modifying WITH queries according to the
semantics that the updates all happen with the same command counter value,
and in an unspecified order.  Therefore one WITH clause can't see the
effects of another, nor can the outer query see the effects other than
through the RETURNING values.  And attempts to do conflicting updates will
have unpredictable results.  We'll need to document all that.

This commit just fixes the code; documentation updates are waiting on
author.

Marko Tiikkaja and Hitoshi Harada

src/include/parser/parse_node.h

@@ -103,6 +103,7 @@ struct ParseState
 	bool		p_hasAggs;
 	bool		p_hasWindowFuncs;
 	bool		p_hasSubLinks;
+	bool		p_hasModifyingCTE;
 	bool		p_is_insert;
 	bool		p_is_update;
 	bool		p_locked_from_parent;