Bring PostOrderCFGView's insert API up to date with other API changes.

r222334 updates LLVM data structure's insert API to return a
pair. This change updates PostOrderCFGView accordingly, so that it can
be used interchangably with other sets.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222445 91177308-0d34-0410-b5e6-96231b3b80d8