MDEV-29666 InnoDB fails to purge secondary index records when indexed virtual columns exist

row_purge_get_partial(): Replaces trx_undo_rec_get_partial_row().
Also copy the purge_node_t::ref to the purge_node_t::row.
In this way, the clustered index key fields will always be
available, even if thanks to
commit d384ead0f0 (MDEV-14799)
they would no longer be repeated in the remaining part of the
undo log record.

mysql-test/suite/gcol/r/innodb_virtual_debug_purge.result

@@ -148,10 +148,7 @@ lock table t write;
 connection prevent_purge;
 commit;
 connection default;
-InnoDB		0 transactions not purged
 disconnect lock_table;
-start transaction with consistent snapshot;
-commit;
 InnoDB		0 transactions not purged
 set global debug_dbug=@old_dbug;
 drop table t;