SI-7590 TreeSet should fail fast if Ordering is null

While migrating scala.tools.nsc.util.TreeSet to
scala.collection.mutable.TreeSet, I messed up initialization order
and realized that TreeSet accepts null as an Ordering and only fails
much later.

This change makes mutable.TreeSet and immutable.TreeSet fail
immediately.
Merge pull request #2681 from soc/SI-7590

SI-7590 TreeSet should fail fast if Ordering is null