Implement polymorphic_allocator<byte> for C++20 (P0339R6)

	* include/std/memory_resource (polymorphic_allocator): Add default
	template argument for C++20.
	(polymorphic_allocator::allocate_bytes)
	(polymorphic_allocator::deallocate_bytes)
	(polymorphic_allocator::allocate_object)
	(polymorphic_allocator::deallocate_object)
	(polymorphic_allocator::new_object)
	(polymorphic_allocator::delete_object): New member functions for
	C++20.
	* testsuite/20_util/polymorphic_allocator/allocate_object.cc: New
	test.

From-SVN: r269362