libstdc++: Fix std::format for floating-point chrono::time_point [PR113500]

Currently trying to use std::format with certain specializations of
std::chrono::time_point is ill-formed, due to one member function of the
__formatter_chrono type which tries to write a time_point to an ostream.
For sys_time<floating-point> or sys_time with a period greater than days
there is no operator<< that can be used.

That operator<< is only needed when using an empty chrono-specs in the
format string, like "{}", but the ill-formed expression gives an error
even if not actually used. This means it's not possible to format some
other specializations of chrono::time_point, even when using a non-empty
chrono-specs.

This fixes it by avoiding using 'os << t' for all chrono::time_point
specializations, and instead using std::format("{:L%F %T}", t). So that
we continue to reject std::format("{}", sys_time{1.0s}) a check for
empty chrono-specs is added to the formatter<sys_time<D>, C>
specialization.

While testing this I noticed that the output for %S with a
floating-point duration was incorrect, as the subseconds part was being
appended to the seconds without a decimal point, and without the correct
number of leading zeros.

libstdc++-v3/ChangeLog:

	PR libstdc++/113500
	* include/bits/chrono_io.h (__formatter_chrono::_M_S): Fix
	printing of subseconds with floating-point rep.
	(__formatter_chrono::_M_format_to_ostream): Do not write
	time_point specializations directly to the ostream.
	(formatter<chrono::sys_time<D>, C>::parse): Do not allow an
	empty chrono-spec if the type fails to meet the constraints for
	writing to an ostream with operator<<.
	* testsuite/std/time/clock/file/io.cc: Check formatting
	non-integral times with empty chrono-specs.
	* testsuite/std/time/clock/gps/io.cc: Likewise.
	* testsuite/std/time/clock/utc/io.cc: Likewise.
	* testsuite/std/time/hh_mm_ss/io.cc: Likewise.