libstdc++: Fix %S format of duration with floating-point rep

I got the order of arguments to std::format_to wrong. It was in a
discarded statement, for a case which wasn't being tested.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_S): Fix order
	of arguments to std::format_to.
	* testsuite/20_util/duration/io.cc: Test subsecond duration with
	floating-point rep.