LWG 4229 points out that the std::ranges::to wording refers to class
types, but I added an assertion using std::is_class_v which only allows
non-union class types. LWG consensus is that unions should be allowed,
so this additionally uses std::is_union_v.
libstdc++-v3/ChangeLog:
* include/std/ranges (ranges::to): Allow unions as well as
non-union class types.
* testsuite/std/ranges/conv/lwg4229.cc: New test.
Reviewed-by: Tomasz KamiĆski <tkaminsk@redhat.com>