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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222447 91177308-0d34-0410-b5e6-96231b3b80d8
