[Bug libstdc++/114244] Need to use round when parsing fractional seconds

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114244

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yup, the seconds part "00.002" is parsed using std::numpunct (in order to
handle the locale's decimal point) and then converted to milliseconds using
duration_cast:

                          auto& __ng = use_facet<num_get<_CharT>>(__loc);
                          long double __val;
                          ios_base::iostate __err2{};
                          __ng.get(__buf, {}, __buf, __err2, __val);
                          if (__is_failed(__err2)) [[unlikely]]
                            __err |= __err2;
                          else
                            {
                              duration<long double> __fs(__val);
                              __s = duration_cast<_Duration>(__fs);
                            }


We also use duration_cast when parsing %c and %X using std::time_get, but
that's an integer conversion there (but the duration_cast should have been
qualified to prevent ADL):

                          __h = hours(__tm.tm_hour);
                          __min = minutes(__tm.tm_min);
                          __s = duration_cast<_Duration>(seconds(__tm.tm_sec));

and another duration_cast in chrono::from_stream for durations. That one could
be used with either integral or floating-point reps.

Do we want to parse "00:00:31" as minutes(1)? I don't think we do, so only the
first case where converting long double to milliseconds should be rounded?