[libc++][chrono] implements UTC clock.

While implementing this feature and its associated LWG issues it turns out
- LWG3316 Correctly define epoch for utc_clock / utc_timepoint
only added non-normative wording to the standard.

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones
- P1361 Integration of chrono with text formatting
- LWG3359 <chrono> leap second support should allow for negative leap seconds