libstdc++: Support link chains in std::chrono::tzdb::locate_zone [PR114770]

Since 2022 the TZif format defined in the zic(8) man page has said that
links can refer to other links, rather than only referring to a zone.
This isn't supported by the C++20 spec, which assumes that the target()
for a chrono::time_zone_link always names a chrono::time_zone, not
another chrono::time_zone_link.

This hasn't been a problem until now, because there are no entries in
the tzdata file that chain links together. However, Debian Sid has
changed the target of the Asia/Chungking link from the Asia/Shanghai
zone to the Asia/Chongqing link, creating a link chain. The libstdc++
code is unable to handle this, so chrono::locate_zone("Asia/Chungking")
will fail with the tzdata.zi file from Debian Sid.

It seems likely that the C++ spec will need a change to allow link
chains, so that the original structure of the IANA database can be fully
represented by chrono::tzdb. The alternative would be for chrono::tzdb
to flatten all chains when loading the data, so that a link's target is
always a zone, but this means throwing away information present in the
tzdata.zi input file.

In anticipation of a change to the spec, this commit adds support for
chained links to libstdc++. When a name is found to be a link, we try to
find its target in the list of zones as before, but now if the target
isn't the name of a zone we don't fail. Instead we look for another link
with that name, and keep doing that until we reach the end of the chain
of links, and then look up the last target as a zone.

This new logic would get stuck in a loop if the tzdata.zi file is buggy
and defines a link chain that contains a cycle, e.g. two links that
refer to each other. To deal with that unlikely case, we use the
tortoise and hare algorithm to detect cycles in link chains, and throw
an exception if we detect a cycle. Cycles in links should never happen,
and it is expected that link chains will be short (if they occur at all)
and so the code is optimized for short chains without cycles. Longer
chains (four or more links) and cycles will do more work, but won't fail
to resolve a chain or get stuck in a loop.

The new test file checks various forms of broken links and cycles.

Also add a new check in the testsuite that every element in the
get_tzdb().zones and get_tzdb().links sequences can be successfully
found using locate_zone.

libstdc++-v3/ChangeLog:

	PR libstdc++/114770
	* src/c++20/tzdb.cc (do_locate_zone): Support links that have
	another link as their target.
	* testsuite/std/time/tzdb/1.cc: Check that all zones and links
	can be found by locate_zone.
	* testsuite/std/time/tzdb/links.cc: New test.