Disallow extended statistics on system columns

Since introduction of extended statistics, we've disallowed references
to system columns. So for example

    CREATE STATISTICS s ON ctid FROM t;

would fail. But with extended statistics on expressions, it was possible
to work around this limitation quite easily

    CREATE STATISTICS s ON (ctid::text) FROM t;

This is an oversight in a4d75c86bf, fixed by adding a simple check.
Backpatch to PostgreSQL 14, where support for extended statistics on
expressions was introduced.

Backpatch-through: 14
Discussion: https://postgr.es/m/20210816013255.GS10479%40telsasoft.com

src/backend/commands/statscmds.c

@@ -288,9 +288,24 @@ CreateStatistics(CreateStatsStmt *stmt)
 			Node	   *expr = selem->expr;
 			Oid			atttype;
 			TypeCacheEntry *type;
+			Bitmapset  *attnums = NULL;
+			int			k;
 
 			Assert(expr != NULL);
 
+			/* Disallow expressions referencing system attributes. */
+			pull_varattnos(expr, 1, &attnums);
+
+			k = -1;
+			while ((k = bms_next_member(attnums, k)) >= 0)
+			{
+				AttrNumber	attnum = k + FirstLowInvalidHeapAttributeNumber;
+				if (attnum <= 0)
+					ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("statistics creation on system columns is not supported")));
+			}
+
 			/*
 			 * Disallow data types without a less-than operator.
 			 *