Add vfs::recursive_directory_iterator

For now, this is only used by its unit tests.  It is similar to the API
in llvm::sys::fs::recursive_directory_iterator, but without some of the
more complex features like requesting that the iterator not recurse into
the next directory, for example.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211732 91177308-0d34-0410-b5e6-96231b3b80d8