Polymorphic Allocators

### Checklist

- [x] I did not find a duplicate of this feature request in the Github Issues section.

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### Description

C++17 introduces new polymorphic allocator support with [`std::polymorphic_allocator`](https://en.cppreference.com/w/cpp/memory/polymorphic_allocator), along with `std::memory_resource` and aliases of all container types as new vocabulary types inside of `std::pmr` namespace.

These utilities are extremely useful for supporting different allocation strategies in a composable way, and would be beneficial to be added to **Backport**

The issue names C++17's std::polymorphic_allocator, std::memory_resource, and std::pmr container aliases, but it does not identify files, tests, or an implementation entry point. Start by locating the existing backported vocabulary types and determine the intended scope; done should mean the requested polymorphic allocator support is implemented with appropriate coverage.

Written by the indexing model from the issue text.