[Modules] Use vfs for (recursive) directory iteration

Clang performs directory walk while searching headers inside modules by
using the ::sys::fs instead of ::vfs. This prevents any code that uses
the VFS (e.g, reproducer scripts) to actually find such headers, since
the VFS will never be searched for those.

Change these places to use vfs::recursive_directory_iterator and
vfs::directory_iterator instead.

Differential Revision: http://reviews.llvm.org/D20266

rdar://problem/25880368

llvm-svn: 269661