Allow referring to functions without arguments when unique

In DDL commands referring to an existing function, allow omitting the
argument list if the function name is unique in its schema, per SQL
standard.

This uses the same logic that the regproc type uses for finding
functions by name only.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/include/nodes/parsenodes.h

@@ -1811,6 +1811,9 @@ typedef struct ObjectWithArgs
 	NodeTag		type;
 	List	   *objname;		/* qualified name of function/operator */
 	List	   *objargs;		/* list of Typename nodes */
+	bool		args_unspecified; /* argument list was omitted, so name must
+								   * be unique (note that objargs == NIL means
+								   * zero args) */
 } ObjectWithArgs;
 
 /*