Maintenance status of poly

Hello

Was wondering about whether folly/poly is being maintained and if PRs would be accepted. From what I've seen its tests are commented out in CI and its source files are unconditionally deleted on Windows builds.

I've been making some minor updates in [a fork](https://github.com/cstratopoulos/folly/tree/feature/poly-update) and I wanted to know if it would be possible to get them merged. In summary:

- [add `noexcept` handling to type traits](https://github.com/cstratopoulos/folly/commit/306965b7255e581074a3b0c8a3bcacfc438e0c7b) fixing #737
- [replace non-type template parameter `auto` detection macro](https://github.com/cstratopoulos/folly/blob/feature/poly-update/folly/detail/PolyDetail.h#L35) since using [`__cpp_template_auto`](https://fossies.org/diffs/gcc/8.2.0_vs_8.3.0/gcc/c-family/ChangeLog-diff.html) is deprecated on GCC and not supported on MSVC
- Some minor changes to SFINAE usage:
- [`getOpsImpl`](https://github.com/facebook/folly/commit/4b746e0e0e61eb1b92c6abf03d5a6318cd05e6e1#diff-510f043e2c48711e0bd189e9a6b4f8cdR677)
- [`MetaDefer`](https://github.com/facebook/folly/commit/4b746e0e0e61eb1b92c6abf03d5a6318cd05e6e1#diff-b84b44a5a85a2415e40fdc8d8ba07106L171)
- [Helper rather than calling `std::get` with tuple inheritance](https://github.com/cstratopoulos/folly/commit/9f2d417714f139c64b6d0f4f246d878bb3725d51), MSVC and clang-cl didn't seem to like this

Note that with the last few bullet points some testing on my local machine indicates that compiling/test passing with recent MSVC (16, definitely, 15.9.x maybe) should be within reach.

The only issue I've encountered is with [non-member functions](https://github.com/facebook/folly/blob/master/folly/docs/Poly.md#non-member-functions-c17) as it seems these require a lambda in an unevaluated context which I think lands in C++20 with [p0315](http://wg21.link/p0315).