libstdc++: Use formatting locale for std::time_put formats

libstdc++: Use formatting locale for std::time_put formats

When using std::time_put to format a chrono value, we should imbue the
formatting locale into the stream. This ensures that when
std::time_put::do_put uses a ctype or __timepunct facet from the locale,
it gets the correct facets.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_locale_fmt):
	Imbue locale into ostringstream.
	* testsuite/std/time/format/localized.cc: Check that correct
	locale is used for call to time_put::put.

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