libstdc++: Fix parsing of fractional seconds [PR114244]

libstdc++: Fix parsing of fractional seconds [PR114244]

When converting a chrono::duration<long double> to a result type with an
integer representation we should use chrono::round<_Duration> so that we
don't truncate towards zero. Rounding ensures that e.g. 0.001999s
becomes 2ms not 1ms.

We can also remove some redundant uses of chrono::duration_cast to
convert from seconds to _Duration, because the _Parser class template
requires _Duration type to be able to represent seconds without loss of
precision.

This also fixes a bug where no fractional part would be parsed for
chrono::duration<long double> because its period is ratio<1>. We should
also consider treat_as_floating_point<rep> when deciding whether to skip
reading a fractional part.

libstdc++-v3/ChangeLog:

	PR libstdc++/114244
	* include/bits/chrono_io.h (_Parser::operator()): Remove
	redundant uses of duration_cast. Use chrono::round to convert
	long double value to durations with integer representations.
	Check represenation type when deciding whether to skip parsing
	fractional seconds.
	* testsuite/20_util/duration/114244.cc: New test.
	* testsuite/20_util/duration/io.cc: Check that a floating-point
	duration with ratio<1> precision can be parsed.