Update documentation to match recent API change. newFrontendActionFactory now returns a unique_ptr.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207789 91177308-0d34-0410-b5e6-96231b3b80d8
Use make_unique<T>(...) rather than unique_ptr<T>(new T(...))

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207792 91177308-0d34-0410-b5e6-96231b3b80d8