libstdc++: Do not require a time-of-day when parsing sys_days [PR114240]

When parsing a std::chrono::sys_days (or a sys_time with an even longer
period) we should not require a time-of-day to be present in the input,
because we can't represent that in the result type anyway.

Rather than trying to decide which specializations should require a
time-of-date and which should not, follow the direction of Howard
Hinnant's date library, which allows extracting a sys_time of any period
from input that only contains a date, defaulting the time-of-day part to
00:00:00. This seems consistent with the intent of the standard, which
says it's an error "If the parse fails to decode a valid date" (i.e., it
doesn't care about decoding a valid time, only a date).

libstdc++-v3/ChangeLog:

	PR libstdc++/114240
	* include/bits/chrono_io.h (_Parser::operator()): Assume
	hours(0) for a time_point, so that a time is not required
	to be present.
	* testsuite/std/time/parse/114240.cc: New test.

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