[VFS] Add level() method to vfs::recursive_directory_iterator

Unlike sys::fs::recursive_directory_iterator,
vfs::recursive_directory_iterator does not implement the level() method,
which tells how deep in the directory tree the current iterator is. This
is needed in the vfs::recursive_directory_iterator so that future
improvements to the crash reproducer will be able to properly access
header for umbrellas when looking into the VFS.

rdar://problem/25880368

llvm-svn: 269520