libstdc++/70940 Start fixing polymorphic memory resources

libstdc++/70940 Start fixing polymorphic memory resources

	PR libstdc++/70940
	* include/experimental/memory_resource
	(__resource_adaptor_imp::do_allocate): Do not default-construct
	rebound allocator.
	(__resource_adaptor_imp::do_deallocate): Likewise. Use
	allocator_traits to get pointer type.
	(__null_memory_resource::do_allocate): Remove unused parameters.
	(__null_memory_resource::do_deallocate): Likewise.
	(__null_memory_resource::do_is_equal): Likewise. Add return statement.
	* testsuite/experimental/type_erased_allocator/1.cc: Combine with ...
	* testsuite/experimental/type_erased_allocator/1_neg.cc: This, and
	move to ...
	* testsuite/experimental/memory_resource/1.cc: Here.
	* testsuite/experimental/memory_resource/null_memory_resource.cc: New.
	* testsuite/experimental/memory_resource/resource_adaptor.cc: New.

From-SVN: r235868