Smaller implementation of to_ordering

Summary:
[Folly] Smaller implementation of `to_ordering`.

Shorter source code, shorter bytecode, no branches/conditionals.

Reviewed By: Orvid

Differential Revision: D9733375

fbshipit-source-id: a2b2a8263b5de2264d3068a424419d63e5b27228

folly/lang/Ordering.h

@@ -22,7 +22,7 @@ enum class ordering : int { lt = -1, eq = 0, gt = 1 };
 
 template <typename T>
 constexpr ordering to_ordering(T c) {
-  return c < T(0) ? ordering::lt : c > T(0) ? ordering::gt : ordering::eq;
+  return ordering(int(c < T(0)) * -1 + int(c > T(0)));
 }
 
 namespace detail {