Make -Wincomplete-umbrella go through the VFS

By using vfs::recursive_directory_iterator, this warning will now fire
when some or all of a module's headers are from VFS mappings.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211746 91177308-0d34-0410-b5e6-96231b3b80d8