[libc++][TZDB] Finishes zoned_time member functions.

Note the implementation of
  zoned_time& operator=(const local_time<Duration>& lt);
is not correct; however the wording cannot be easily implemented. It could
be if the object caches the local_time assigned. However this does not
seem to intended. The current implementation matches MSVC STL and
libstdc++.

Implements parts of:
- P0355 Extending to chrono Calendars and Time Zones