This patch is the rebase and squash of three earlier patches. It supersedes all three of them.
- D47111: experimental monotonic_buffer_resource.
- D47358: experimental pool resources.
- D47360: Copy std::experimental::pmr to std::pmr.
The significant difference between this patch and the-sum-of-those-three is that this patch does not add std::experimental::pmr::monotonic_buffer_resource and so on. This patch simply adds the C++17 standard facilities, and leaves the std::experimental namespace entirely alone.