libstdc++: Fix std::chrono::parse for TAI and GPS clocks

libstdc++: Fix std::chrono::parse for TAI and GPS clocks

Howard Hinnant brought to my attention that chrono::parse was giving
incorrect values for chrono::gps_clock, because it was applying the
offset between the GPS clock and UTC. That's incorrect, because when we
parse HH::MM::SS as a GPS time, the result should be that time, not
HH:MM:SS+offset.

The problem was that I was using clock_cast to convert from sys_time to
utc_time and then using clock_time again to convert to gps_time. The
solution is to convert the parsed time into an duration representing the
time since the GPS clock's epoch, then construct a gps_time directly
from that duration.

As well as adding tests for correct round tripping of times for all
clocks, this also adds some more tests for correct results with
std::format.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (from_stream): Fix conversions in
	overloads for gps_time and tai_time.
	* testsuite/std/time/clock/file/io.cc: Test round tripping using
	chrono::parse. Add additional std::format tests.
	* testsuite/std/time/clock/gps/io.cc: Likewise.
	* testsuite/std/time/clock/local/io.cc: Likewise.
	* testsuite/std/time/clock/tai/io.cc: Likewise.
	* testsuite/std/time/clock/utc/io.cc: Likewise.