Fix array coercion expressions to ensure that the correct volatility is

seen by code inspecting the expression.  The best way to do this seems
to be to drop the original representation as a function invocation, and
instead make a special expression node type that represents applying
the element-type coercion function to each array element.  In this way
the element function is exposed and will be checked for volatility.
Per report from Guillaume Smet.

src/include/nodes/nodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.196 2007/02/20 17:32:17 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.197 2007/03/27 23:21:12 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -121,6 +121,7 @@ typedef enum NodeTag
 	T_FieldSelect,
 	T_FieldStore,
 	T_RelabelType,
+	T_ArrayCoerceExpr,
 	T_ConvertRowtypeExpr,
 	T_CaseExpr,
 	T_CaseWhen,
@@ -159,6 +160,7 @@ typedef enum NodeTag
 	T_SubPlanState,
 	T_FieldSelectState,
 	T_FieldStoreState,
+	T_ArrayCoerceExprState,
 	T_ConvertRowtypeExprState,
 	T_CaseExprState,
 	T_CaseWhenState,