libstdc++: Fix format call and test formatting with empty specs for durations.

libstdc++: Fix format call and test formatting with empty specs for durations.

This patches fixes an obvious error, where the output iterator argument was
missing for call to format_to, when duration with custom representation types
are used.

It's also adding the test for behavior of ostream operator and the formatting
with empty chron-spec for the chrono types. Current coverage is:
 * duration and hh_mm_ss in this commit,
 * calendar types in r16-1016-g28a17985dd34b7.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono:_M_s): Add missing
	__out argument to format_to call.
	* testsuite/std/time/format/empty_spec.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>