libstdc++: Fix std::format of chrono::local_days with {} [PR120293]

libstdc++: Fix std::format of chrono::local_days with {} [PR120293]

Formatting of chrono::local_days with an empty chrono-specs should be
equivalent to inserting it into an ostream, which should use the
overload for inserting chrono::sys_days into an ostream. The
implementation of empty chrono-specs in _M_format_to_ostream takes some
short cuts, and that wasn't being done correctly for chrono::local_days.

libstdc++-v3/ChangeLog:

	PR libstdc++/120293
	* include/bits/chrono_io.h (_M_format_to_ostream): Add special
	case for local_time convertible to local_days.
	* testsuite/std/time/clock/local/io.cc: Check formatting of
	chrono::local_days.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>