libstdc++: Add std::is_layout_compatible trait for C++20
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
* include/std/type_traits (is_layout_compatible): Define.
(is_corresponding_member): Define.
* include/std/version (__cpp_lib_is_layout_compatible): Define.
* testsuite/20_util/is_layout_compatible/is_corresponding_member.cc:
New test.
* testsuite/20_util/is_layout_compatible/value.cc: New test.
* testsuite/20_util/is_layout_compatible/version.cc: New test.
* testsuite/20_util/is_pointer_interconvertible/with_class.cc:
New test.
* testsuite/23_containers/span/layout_compat.cc: Do not use real
std::is_layout_compatible trait if available.