libstdc++: Fix formatter for low-resolution chrono::zoned_time (LWG 4124)

This implements the proposed resolution of LWG 4124, so that
low-resolution chrono::zoned_time objects can be formatted. The
formatter for zoned_time<D, P> needs to account for get_local_time
returning local_time<common_type_t<D, seconds>> not local_time<D>.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__local_time_fmt_for): New alias
	template.
	(formatter<zoned_time<D, P>>): Use __local_time_fmt_for.
	* testsuite/std/time/zoned_time/io.cc: Check zoned_time<minutes>
	can be formatted.