Fix LATERAL references to target table of UPDATE/DELETE.

I failed to think much about UPDATE/DELETE when implementing LATERAL :-(.
The implemented behavior ended up being that subqueries in the FROM or
USING clause (respectively) could access the update/delete target table as
though it were a lateral reference; which seems fine if they said LATERAL,
but certainly ought to draw an error if they didn't.  Fix it so you get a
suitable error when you omit LATERAL.  Per report from Emre Hasegeli.

src/backend/parser/parse_relation.c

@@ -1730,6 +1730,10 @@ isLockedRefname(ParseState *pstate, const char *refname)
  * and/or namespace list.  (We assume caller has checked for any
  * namespace conflicts.)  The RTE is always marked as unconditionally
  * visible, that is, not LATERAL-only.
+ *
+ * Note: some callers know that they can find the new ParseNamespaceItem
+ * at the end of the pstate->p_namespace list.  This is a bit ugly but not
+ * worth complicating this function's signature for.
  */
 void
 addRTEtoQuery(ParseState *pstate, RangeTblEntry *rte,