LWG 2969 "polymorphic_allocator::construct() shouldn't pass resource()"

Patch from Arthur O'Dwyer.

In the TS, `uses_allocator` construction for `pair` tried to use an allocator
type of `memory_resource*`, which is incorrect because `memory_resource*` is
not an allocator type. LWG 2969 fixed it to use `polymorphic_allocator` as the
allocator type instead.

https://wg21.link/lwg2969

(D47090 included this in `<memory_resource>`; at Eric's request, I've split
this out into its own patch applied to the existing
`<experimental/memory_resource>` instead.)

Reviewed as https://reviews.llvm.org/D47109

llvm-svn: 333384