PR72792 PR72793 relax requirements on rebind members

PR72792 PR72793 relax requirements on rebind members

	PR libstdc++/72792
	PR libstdc++/72793
	* include/bits/alloc_traits.h (__allocator_traits_base::__rebind):
	Replace with class template using void_t.
	(__alloc_rebind): Define in terms of
	__allocator_traits_base::__rebind.
	(allocator_traits): Remove unconditional static_assert for
	rebind_alloc.
	* include/bits/ptr_traits.h (__replace_first_arg): Remove type member.
	(pointer_traits::__rebind): Replace with class template using void_t.
	(pointer_traits::rebind): Define in terms of __rebind.
	(pointer_traits): Remove unconditional static_assert for rebind.
	* testsuite/20_util/allocator_traits/members/rebind_alloc.cc: New test.
	* testsuite/20_util/pointer_traits/rebind.cc: New test.

From-SVN: r244680