libstdc++: Fix parsing of leap seconds as chrono::utc_time [PR114279]

Implementing all chrono::from_stream overloads in terms of
chrono::sys_time meant that a leap second time like 23:59:60.001 cannot
be parsed, because that cannot be represented in a sys_time.

The fix to support parsing leap seconds as utc_time is to convert the
parsed date to utc_time<days> and then add the parsed time to that,
which allows the result to land in a leap second, rather than doing all
the arithmetic with sys_time which doesn't have leap seconds.

For local_time we also allow %S to parse a 60s value, because doing
otherwise might disallow some valid uses. We can't know all use cases
users have for treating times as local_time.

For all other clocks, we can reject times that have 60 or 60.nnn as the
seconds part, because that cannot occur in a valid UNIX, GPS, or TAI
time. Since our chrono::file_clock uses sys_time, it can't occur for
that clock either.

In order to support this a new _M_is_leap_second member is needed in the
_Parser type. This can be added at the end, where most targets currently
have padding bytes. Similar to what I did recently for formatter _Spec
structs, we can also reserve additional padding bits for future
expansion.

This also fixes bugs in the from_stream overloads for utc_time,
tai_time, gps_time, and file_time, which were not using time_point_cast
to explicitly convert to the result type. That's needed because the
result type might have lower precision than the value returned from
from_sys or from_utc, which has a precision no lower than seconds.

libstdc++-v3/ChangeLog:

	PR libstdc++/114279
	* include/bits/chrono_io.h (_Parser::_M_is_leap_second): New
	data member.
	(_Parser::_M_reserved): Reserve padding bits for future use.
	(_Parser::operator()): Set _M_is_leap_second if %S reads 60s.
	(from_stream): Only allow _M_is_leap_second for utc_time and
	local_time. Adjust arithmetic for utc_time so that leap seconds
	are preserved. Use time_point_cast to convert to a possibly
	lower-precision result type.
	* testsuite/std/time/parse.cc: Move to ...
	* testsuite/std/time/parse/parse.cc: ... here.
	* testsuite/std/time/parse/114279.cc: New test.

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