Update template specialization to reflect API changes.

po_iterator_storage's insertEdge was updated to reflect the API
changes from many of our insert methods in r222334, however the
template specialization for external storage was not updated. This
updates the specialization.
Compilation test for PostOrderIterator.

If the template specialization for externally managed sets in
PostOrderIterator call too far out of sync with each other, this unit
test will fail to build. This is especially useful for developers who
may not build Clang (the only in-tree user) every time.