This is a partial port of my patch in MySQL 8.0.
In MySQL 8.0, all InnoDB references to DBUG_OFF were replaced
with UNIV_DEBUG. We will not do that in MariaDB.
InnoDB used two independent compile-time flags that distinguish
debug and non-debug builds, which is confusing.
Also, make ut_ad() and alias of DBUG_ASSERT().
