libstdc++: Add std::format support to <chrono>

libstdc++: Add std::format support to <chrono>

This adds the operator<< overloads and std::formatter specializations
required by C++20 so that <chrono> types can be written to ostreams and
printed with std::format.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/std/chrono (operator<<): Move to new header.
	(nonexistent_local_time::_M_make_what_str): Define correctly.
	(ambiguous_local_time::_M_make_what_str): Likewise.
	* include/bits/chrono_io.h: New file.
	* src/c++20/tzdb.cc (operator<<(ostream&, const Rule&)): Use
	new ostream output for month and weekday types.
	* testsuite/20_util/duration/io.cc: Test std::format support.
	* testsuite/std/time/exceptions.cc: Check what() strings.
	* testsuite/std/time/syn_c++20.cc: Uncomment local_time_format.
	* testsuite/std/time/time_zone/get_info_local.cc: Enable check
	for formatted output of local_info objects.
	* testsuite/std/time/clock/file/io.cc: New test.
	* testsuite/std/time/clock/gps/io.cc: New test.
	* testsuite/std/time/clock/system/io.cc: New test.
	* testsuite/std/time/clock/tai/io.cc: New test.
	* testsuite/std/time/clock/utc/io.cc: New test.
	* testsuite/std/time/day/io.cc: New test.
	* testsuite/std/time/format.cc: New test.
	* testsuite/std/time/hh_mm_ss/io.cc: New test.
	* testsuite/std/time/month/io.cc: New test.
	* testsuite/std/time/weekday/io.cc: New test.
	* testsuite/std/time/year/io.cc: New test.
	* testsuite/std/time/year_month_day/io.cc: New test.