Summary:
C++17 defined integer traits for `__int128` as well as a (trivial) std::hash,
this diff adds the check in `FOLLY_SUPPLY_MISSING_INT128_TRAITS`.

There was an unintended linebreak in the cmake test which rendered it non
functional. This diff fixes it.

Original issue:
https://github.com/facebook/folly/commit/b2b96f59c57cd7aba686cc4f61ceba0314cdeb72#comments

Reviewed By: yfeldblum

Differential Revision: D7138006

fbshipit-source-id: ca1841b9ca847f1f6570b58e95380f50fe092345