Fix possible UB in constexpr_strcmp

Summary: [Folly] Fix possible UB in `constexpr_strcmp` since signed arithmetic wraparound is undefined.

Differential Revision: D23435902

fbshipit-source-id: f71103f58f9a1a69dbc7fa462703605d1fc78348

folly/portability/Constexpr.h

@@ -44,7 +44,7 @@ constexpr int constexpr_strcmp_fallback(const Char* s1, const Char* s2) {
   while (*s1 && *s1 == *s2) {
     ++s1, ++s2;
   }
-  return static_cast<int>(*s1 - *s2);
+  return int(*s2 < *s1) - int(*s1 < *s2);
 }
 
 } // namespace detail