Summary:
[Folly] `constexpr_abs`.

Is `constexpr`.

Works over integral and floating types.

If given an integral type, the return type is the usigned version of that integral type, thereby avoiding the undefined behavior of `std::abs(std::numeric_limits<int>::min())`.

Reviewed By: simpkins

Differential Revision: D3654072

fbshipit-source-id: 24fefc0c3b055f78ba3e07472c38fb9c550e0f31