Fix HashSet<>-related parts of #491858, #517415

Fix HashSet<>-related parts of #491858, #517415

* System.Collections.Generic/HashSet.cs (Enumerator.next): Rename from
Enumerator.current and change meaning to be the index of the slot to start
searching on the next iteration.
(Enumerator.current): New.  Contains the current element.
(Enumerator.MoveNext): Update.  Avoid an out-of-bounds access.
(Enumerator.Current): Don't check any invariants.
* Test/System.Collections.Generic/HashSetTest.cs (Enumerate_Current):
New test, including cases from #491858, #517415.

svn path=/trunk/mcs/; revision=139105