libstdc++: Resolve UNTIL save adjustment at tzdb loading time [PR116110]

This patch moves the save calculation (ZoneInfo::calc_save) to database
loading code (reload_tzdb) instead of applying it on demand when zone
is queried (time_zone::_M_get_sys_info). This eliminates the performance
impact on non-first calls (that return the cached result), caused by
iterator adjustment checks.

Local performance test indicate a 10% cost (30ns to 33ns on average) for
cached queries with on-demand implementation (after r17-2466-g020e02fcf28),
combined with huge swings on time on first calls. This patch leads 200ms
increase (1.95s to 2.15s) on time of reload_tzdb, that happens only during
initial load (and later explicit reload).

As we need two bits of state (expanded or until_pending), I have decided
to keep the four value m_state enum.

	PR libstdc++/116110

libstdc++-v3/ChangeLog:

	* src/c++20/tzdb.cc (time_zone::_M_get_sys_info): Remove
	ZoneInfo::calc_save invocation and related iterator adjustment.
	(chrono::reload_tzdb): Calculate save (invoke calc_save) for
	all infos on all zones.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>