[RecursiveASTVisitor] Improve post-order traversal unit test
[RecursiveASTVisitor] Improve post-order traversal unit test

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@288976 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit b9cb1c8a1ebf52695372de12c7b04c8ef1bd8b4e)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang