Allow SET STATISTICS on expression indexes

Index columns are referenced by ordinal number rather than name, e.g.
CREATE INDEX coord_idx ON measured (x, y, (z + t));
ALTER INDEX coord_idx ALTER COLUMN 3 SET STATISTICS 1000;

Incompatibility note for release notes:
\d+ for indexes now also displays Stats Target

Authors: Alexander Korotkov, with contribution by Adrien NAYRAT
Review: Adrien NAYRAT, Simon Riggs
Wordsmith: Simon Riggs

src/include/nodes/parsenodes.h

@@ -1777,6 +1777,8 @@ typedef struct AlterTableCmd	/* one subcommand of an ALTER TABLE */
 	AlterTableType subtype;		/* Type of table alteration to apply */
 	char	   *name;			/* column, constraint, or trigger to act on,
 								 * or tablespace */
+	int16		num;			/* attribute number for columns referenced
+								 * by number */
 	RoleSpec   *newowner;
 	Node	   *def;			/* definition of new column, index,
 								 * constraint, or parent table */