Summary:
- Backporting `std::index_sequence`, `std::integer_sequence`, and other
  associated template aliases such as `index_sequence_for` and
  `make_index_sequence` is not needed as Folly requires C++14. These
  functions and template aliases are available in C++14 standard libraries,
  including GCC 4.9.
Pull Request resolved: https://github.com/facebook/folly/pull/1022

Reviewed By: Orvid

Differential Revision: D14119891

Pulled By: yfeldblum

fbshipit-source-id: 5ba55cc91495cf488cebfa1ebe82c0ff919e9840