libstdc++: Fix rounding in chrono::parse

I noticed that chrono::parse was using duration_cast and time_point_cast
to convert the parsed value to the result. Those functions truncate
towards zero, which is not generally what you want. Especially for
negative times before the epoch, where truncating towards zero rounds
"up" towards the next duration/time_point. Using chrono::round is
typically better, as that rounds to nearest.

However, while testing the fix I realised that rounding to the nearest
can give surprising results in some cases. For example if we parse a
chrono::sys_days using chrono::parse("F %T", "2024-09-22 18:34:56", tp)
then we will round up to the next day, i.e. sys_days(2024y/09/23). That
seems surprising, and I think 2024-09-22 is what most users would
expect.

This change attempts to provide a hybrid rounding heuristic where we use
chrono::round for the general case, but when the result has a period
that is one of minutes, hours, days, weeks, or years then we truncate
towards negative infinity using chrono::floor. This means that we
truncate "2024-09-22 18:34:56" to the start of the current
minute/hour/day/week/year, instead of rounding up to 2024-09-23, or to
18:35, or 17:00. For a period of months chrono::round is used, because
the months duration is defined as a twelfth of a year, which is not
actually the length of any calendar month. We don't want to truncate to
a whole number of "months" if that can actually go from e.g. 2023-03-01
to 2023-01-31, because February is shorter than chrono::months(1).

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__detail::__use_floor): New
	function.
	(__detail::__round): New function.
	(from_stream): Use __detail::__round.
	* testsuite/std/time/clock/file/io.cc: Check for expected
	rounding in parse.
	* testsuite/std/time/clock/gps/io.cc: Likewise.

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