libstdc++: Define std::is_pointer_interconvertible_base_of for C++20

libstdc++: Define std::is_pointer_interconvertible_base_of for C++20

Implement these traits using the new built-ins that Jakub added
recently.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/std/type_traits (__cpp_lib_is_pointer_interconvertible)
	(is_pointer_interconvertible_base_of_v)
	(is_pointer_interconvertible_base_of): Define for C++20.
	* include/std/version (__cpp_lib_is_pointer_interconvertible):
	Define.
	* testsuite/23_containers/span/layout_compat.cc: Use correct
	feature test macro for std::is_layout_compatible_v.
	* testsuite/20_util/is_pointer_interconvertible/value.cc: New test.
	* testsuite/20_util/is_pointer_interconvertible/version.cc: New test.