folly: fix make_optional compliation issue with gnu++17
folly: fix make_optional compliation issue with gnu++17

Summary:
This clashes with std::make_optional with -std=gnu++17 giving:
  call of overloaded 'make_optional(std::__cxx11::basic_string<char>)' is ambiguous

Reviewed By: yfeldblum

Differential Revision: D6780579

fbshipit-source-id: fea8c76869e4d9b744fc3182379328a91879b1d2

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