libstdc++: Fix formatting of chrono::duration with character rep [PR116755]

Implement Peter Dimov's suggestion for resolving LWG 4118, which is to
use +d.count() so that character types are promoted to an integer type
before formatting them. This didn't have unanimous consensus in the
committee as Howard Hinnant proposed that we should format the rep
consistently with std::format("{}", d.count()) instead. That ends up
being more complicated, because it makes std::formattable a precondition
of operator<< which was not previously the case, and it means that
ios_base::fmtflags from the stream would be ignored because std::format
doesn't use them.

libstdc++-v3/ChangeLog:

	PR libstdc++/116755
	* include/bits/chrono_io.h (operator<<): Use +d.count() for
	duration inserter.
	(__formatter_chrono::_M_format): Likewise for %Q format.
	* testsuite/20_util/duration/io.cc: Test durations with
	character types as reps.