libstdc++: Fix formatting of most negative chrono::duration [PR116755]

When formatting chrono::duration<signed-integer-type, P>::min() we were
causing undefined behaviour by trying to form the negative of the most
negative value. If we convert negative durations with integer rep to the
corresponding unsigned integer rep then we can safely represent all
values.

libstdc++-v3/ChangeLog:

	PR libstdc++/116755
	* include/bits/chrono_io.h (formatter<duration<R,P>>::format):
	Cast negative integral durations to unsigned rep.
	* testsuite/20_util/duration/io.cc: Test the most negative
	integer durations.