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.

Tested powerpc64le-linux. Committed to trunk.