Summary:
[Folly] Fix incorrect `forward_tuple` test which only passes with libc++ < 3.8 and libstdc++ < 8.

The test would fail with libc++ >= 3.8 and with libstdc++ >= 8.

Spec fix in: https://cplusplus.github.io/LWG/issue2485.
Upstream libc++ fix in: https://github.com/llvm-mirror/libcxx/commit/199bee0ea7750b8e96a25414c3c2a7b465336e36.
Upstream libstdc++ fix in: https://github.com/gcc-mirror/gcc/commit/6d8efcd444067c046d72ac8e11737230f031a031.

Closes #1078.

Reviewed By: pixelb

Differential Revision: D14590323

fbshipit-source-id: ff023ab844d5da97e83f09576897304ac06678b8