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
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333384 91177308-0d34-0410-b5e6-96231b3b80d8
