Alisdair noticed that __cpp_lib_polymorphic_allocator is defined in
<memory> but std::polymoprhic_allocator is defined in <memory_resource>
(which is not included by <memory>).

This seems like a mistake in P1902R1, as I can't see any reason to put it
in a different header from the feature it describes.

I'll submit a new LWG issue for this.