libstdc++: Add C++20 clocks

Also add the basic types for timezones, without the non-inline
definitions needed to actually use them.

The get_leap_second_info function currently uses a hardcoded list of
leap seconds, correct as of the end of 2022. That needs to be replaced
with a dynamically generated list read from the system tzdata. That will
be done in a later patch.

libstdc++-v3/ChangeLog:

	* include/std/chrono (utc_clock, tai_clock, gps_clock): Define.
	(clock_time_conversion, clock_cast): Define.
	(sys_info, local_info): Define structs for timezone information.
	(nonexistent_local_time, ambiguous_local_time): Define
	exceptions for invalid times.
	(time_zone, time_zone_link, leap_second, zoned_traits, tzdb)
	(tzdb_list): Define classes representing time zones.
	(get_leap_second_info): Define new function returning leap
	second offset for a given time point.
	* testsuite/std/time/clock/gps/1.cc: New test.
	* testsuite/std/time/clock/tai/1.cc: New test.
	* testsuite/std/time/clock/utc/1.cc: New test.