Add directory_iterator for (non-recursive) iteration of VFS directories

The API is based on sys::fs::directory_iterator, but it allows iterating
over overlays and the yaml-based VFS.  For now, it isn't used by
anything (except its tests).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211623 91177308-0d34-0410-b5e6-96231b3b80d8