[libcxx] Fix the preexisting directory_iterator code for windows

The directory_iterator.cpp file did contain an incomplete,
non-working implementation for windows.

Change it to use the wchar version of the APIs.

Don't set the windows specific errors from GetLastError() as code
in the generic category; remap the errors to the std::errc values.

Error out cleanly on empty paths.

Invoke FindFirstFile on <directoryname>/* to actually list the
entries of the directory.

If the first entry retured by FindFirstFile is to be skipped (e.g.
being "." or ".."), call advance() (which calls FindNextFile and loops)
which doesn't return until a valid entry is found (or the end is
reached).

Differential Revision: https://reviews.llvm.org/D91140