There is no difference between the previous IgnoreMessage() function
and a null std::function, from functionality's perspective.
The user can set nullptr as the MessageConsumer, so need to guard
against nullptr before calling the consumer anyway. It's better
we use it internally so that it may expose problems by us instead
of the user.
Default-constructed Pass/PassManager will have a MessageConsumer
which ignores all messages. SetMessageConsumer() should be called
to supply a meaningful MessageConsumer.
