This decoupling allows in further versions of MySQL enum interval_type to
  be reordered without this affecting any backward compatibility in the
  events code.
  This changeset doesn't change any exposed behavior but makes events' code
  more durable to changes outside of their code base.
  
  To the reviewer: There is no regression test included as it is impossible
  to construct one with the current infrastructure which can test it. To test
  the code one has create and event, then change the order of
  enum interval_type in my_time.h, update sql/time.cc, recompile the server
  and run it with scheduler running.