libstdc++: Resolve named-rule UNTIL save adjustment [PR116110]

The previous patches for PR 116110 left one case unresolved: a Zone
line with a wall-time UNTIL whose RULES field is a named rule set.
The save value used to convert the wall UNTIL to UTC depends on which
rule of the set was active at the UNTIL instant, but at parse time
the rule records have not all been loaded, so the active-rule lookup
can't yet be performed.

The remaining FIXME in operator>>(istream&, ZoneInfo&) caused zones
like Africa/Algiers (around 1977-10-21) to place their zone-line
boundary one save-period off from the canonical zic interpretation,
producing brief incorrect sys_info windows during DST transitions.

This commit defers the save adjustment to _M_get_sys_info call,
and applies it only for the ZoneInfo surrounding the specified
time. This assumes that applying the save does not change relative
order of ZoneInfo (i.e. the until members are spaced more than save).

The m_expanded member is replaced with four state _M_state setting:
Expanded, and three separte state used for rule based zones:
* SaveKnown - m_save contains save value at time m_until
* SavePending - m_save is defualted to zero, m_until is correct
* UntilPending - as above, but also m_until time need to adjusted
The parser set the UntilPending state when it sees a wall-time
UNTIL on a named-rule line, and SaveKnown if not UNTIL date is
specified.

The fixup in _M_get_sys_info, is performed by calc_save function
on previous/current zone. If necessary (depending on m_state) this
function updates m_save and m_until members, and returns true
if the m_until was shifted in the process. In such case, we redo
the comparision of until() against tp, and move iterator to active
zone info if necessary. The save at boundary for previous/current
ZoneInfo is required to implement proper zone merging, so it is
always computed.

The active-rule lookup either reuses the existing find_active_rule
overload accepting sys_seconds (if UNTIL is not affected by save),
or newly introduced overload accepting local_seconds (local time).
The local_seconds versions, follows the same logic for finding
rule transitions (extracted to find_surrounding_transitions)
surrounding the sys_time. The final active rule, is then determined
after converting the transition times to local time, including the
running save.

The test_apia case in 116110.cc had a hardcoded `+11h` workaround
for the unfixed bug; with this fix in place the workaround is removed
and the value becomes the canonical `+10h`.

libstdc++-v3/ChangeLog:

	PR libstdc++/116110
	* src/c++20/tzdb.cc (ZoneInfo::calc_save, ZoneInfo::State): Define.
	(ZoneInfo::m_expanded): Replaced with m_state.
	(ZoneInfo::m_pos): Reduce the bit with to 14.
	(ZoneInfo::m_state): Expands m_expanded with four state enum.
	(ZoneInfo::ZoneInfo, ZoneInfo::expanded, ZoneInfo::set_abbrev):
	Replace m_expanded usage with m_state and State::Expanded.
	(Transitions, find_surrounding_transitions): Extracted from
	find_active_rule.
	(find_active_rule): Define overload for local_seconds. Implement
	both in terms of find_surrounding_transitions.
	(time_zone::_M_get_sys_info): Use ZoneInfo::calc_save to adjust
	until() on surrounding zones, and shift the current zone iterator
	(i) accordingly.
	(operator>>(istream&, ZoneInfo&)): Set m_state for UntilPending
	for UNTIL time using wall time, and SaveKnown for lines without
	end date. For clarity, set m_save to 0 if daylight is not used.
	* testsuite/std/time/time_zone/116110.cc (test_apia): Use correct
	value of the the total offset.
	testsuite/std/time/time_zone/pr116110_named.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Co-authored-by: Álvaro Begué <alvaro.begue@gmail.com>
Signed-off-by: Álvaro Begué <alvaro.begue@gmail.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>