Create new ParseExprKind for use by policy expressions.

Policy USING and WITH CHECK expressions were using EXPR_KIND_WHERE for
parse analysis, which results in inappropriate ERROR messages when
the expression contains unsupported constructs such as aggregates.
Create a new ParseExprKind called EXPR_KIND_POLICY and tailor the
related messages to fit.

Reported by Noah Misch. Reviewed by Dean Rasheed, Alvaro Herrera,
and Robert Haas. Back-patch to 9.5 where RLS was introduced.

src/include/parser/parse_node.h

@@ -63,7 +63,8 @@ typedef enum ParseExprKind
 	EXPR_KIND_INDEX_PREDICATE,	/* index predicate */
 	EXPR_KIND_ALTER_COL_TRANSFORM,		/* transform expr in ALTER COLUMN TYPE */
 	EXPR_KIND_EXECUTE_PARAMETER,	/* parameter value in EXECUTE */
-	EXPR_KIND_TRIGGER_WHEN		/* WHEN condition in CREATE TRIGGER */
+	EXPR_KIND_TRIGGER_WHEN,		/* WHEN condition in CREATE TRIGGER */
+	EXPR_KIND_POLICY			/* USING or WITH CHECK expr in policy */
 } ParseExprKind;