This is the largest missing piece of C++20 support. Only the cxx11 ABI
is supported, due to the use of std::string in the API for time zones.
For the old gcc4 ABI, utc_clock and leap seconds are supported, but only
using a hardcoded list of leap seconds, no up-to-date tzdb::leap_seconds
information is available, and no time zones or zoned_time conversions.

The implementation currently depends on a tzdata.zi file being provided
by the OS or the user. The expected location is /usr/share/zoneinfo but
that can be changed using --with-libstdcxx-zoneinfo-dir=PATH. On targets
that support it there is also a weak symbol that users can override in
their own program (which also helps with testing):

extern "C++" const char* __gnu_cxx::zoneinfo_dir_override();

If no file is found, a fallback tzdb object will be created which only
contains the "Etc/UTC" and "Etc/GMT" time zones.

A leapseconds file is also expected in the same directory, but if that
isn't present then a hardcoded list of leapseconds is used, which is
correct at least as far as 2023-06-28 (and it currently looks like no
leap second will be inserted for a few years).

The tzdata.zi and leapseconds files from https://www.iana.org/time-zones
are in the public domain, so shipping copies of them with GCC would be
an option. However, the tzdata.zi file will rapidly become outdated, so
users should really provide it themselves (or convince their OS vendor
to do so). It would also be possible to implement an alternative parser
for the compiled tzdata files (one per time zone) under
/usr/share/zoneinfo. Those files are present on more operating systems,
but do not contain all the information present in tzdata.zi.
Specifically, the "links" are not present, so that e.g. "UTC" and
"Universal" are distinct time zones, rather than both being links to the
canonical "Etc/UTC" zone. For some platforms those files are hard links
to the same file, but there's no indication which zone is the canonical
name and which is a link. Other platforms just store them in different
inodes anyway. I do not plan to add such an alternative parser for the
compiled files. That would need to be contributed by maintainers or
users of targets that require it, if making tzdata.zi available is not
an option. The library ABI would not need to change for a new tzdb
implementation, because everything in tzdb_list, tzdb and time_zone is
implemented as a pimpl (except for the shared_ptr links between nodes,
described below). That means the new exported symbols added by this
commit should be stable even if the implementation is completely
rewritten.

The information from tzdata.zi is parsed and stored in data structures
that closely model the info in the file. This is a space-efficient
representation that uses less memory that storing every transition for
every time zone.  It also avoids spending time expanding that
information into time zone transitions that might never be needed by the
program.  When a conversion to/from a local time to UTC is requested the
information will be processed to determine the time zone transitions
close to the time being converted.

There is a bug in some time zone transitions. When generating a sys_info
object immediately after one that was previously generated, we need to
find the previous rule that was in effect and note its offset and
letters. This is so that the start time and abbreviation of the new
sys_info will be correct. This only affects time zones that use a format
like "C%sT" where the LETTERS replacing %s are non-empty for standard
time, e.g. "Asia/Shanghai" which uses "CST" for standard time and "CDT"
for daylight time.

The tzdb_list structure maintains a linked list of tzdb nodes using
shared_ptr links. This allows the iterators into the list to share
ownership with the list itself. This offers a non-portable solution to a
lifetime issue in the API. Because tzdb objects can be erased from the
list using tzdb_list::erase_after, separate modules/libraries in a large
program cannot guarantee that any const tzdb& or const time_zone*
remains valid indefinitely. Holding onto a tzdb_list::const_iterator
will extend the tzdb object's lifetime, even if it's erased from the
list. An alternative design would be for the list iterator to hold a
weak_ptr. This would allow users to test whether the tzdb still exists
when the iterator is dereferenced, which is better than just having a
dangling raw pointer. That doesn't actually extend the tzdb's lifetime
though, and every use of it would need to be preceded by checking the
weak_ptr. Using shared_ptr adds a little bit of overhead but allows
users to solve the lifetime issue if they rely on the libstdc++-specific
iterator property.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ZONEINFO_DIR): New macro.
	* config.h.in: Regenerate.
	* config/abi/pre/gnu.ver: Export new symbols.
	* configure: Regenerate.
	* configure.ac (GLIBCXX_ZONEINFO_DIR): Use new macro.
	* include/std/chrono (utc_clock::from_sys): Correct handling
	of leap seconds.
	(nonexistent_local_time::_M_make_what_str): Define.
	(ambiguous_local_time::_M_make_what_str): Define.
	(__throw_bad_local_time): Define new function.
	(time_zone, tzdb_list, tzdb): Implement all members.
	(remote_version, zoned_time, get_leap_second_info): Define.
	* include/std/version: Add comment for __cpp_lib_chrono.
	* src/c++20/Makefile.am: Add new file.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++20/tzdb.cc: New file.
	* testsuite/lib/libstdc++.exp: Define effective target tzdb.
	* testsuite/std/time/clock/file/members.cc: Check file_time
	alias and file_clock::now() member.
	* testsuite/std/time/clock/gps/1.cc: Likewise for gps_clock.
	* testsuite/std/time/clock/tai/1.cc: Likewise for tai_clock.
	* testsuite/std/time/syn_c++20.cc: Uncomment everything except
	parse.
	* testsuite/std/time/clock/utc/leap_second_info.cc: New test.
	* testsuite/std/time/exceptions.cc: New test.
	* testsuite/std/time/time_zone/get_info_local.cc: New test.
	* testsuite/std/time/time_zone/get_info_sys.cc: New test.
	* testsuite/std/time/time_zone/requirements.cc: New test.
	* testsuite/std/time/tzdb/1.cc: New test.
	* testsuite/std/time/tzdb/leap_seconds.cc: New test.
	* testsuite/std/time/tzdb_list/1.cc: New test.
	* testsuite/std/time/tzdb_list/requirements.cc: New test.
	* testsuite/std/time/zoned_time/1.cc: New test.
	* testsuite/std/time/zoned_time/custom.cc: New test.
	* testsuite/std/time/zoned_time/deduction.cc: New test.
	* testsuite/std/time/zoned_time/req_neg.cc: New test.
	* testsuite/std/time/zoned_time/requirements.cc: New test.
	* testsuite/std/time/zoned_traits.cc: New test.