Add a note into constexpr_strcmp

Summary: This explains why the commonly used `int(*s1 - *s2)` was changed to a slightly less obvious alternative.

Reviewed By: yfeldblum

Differential Revision: D23537394

fbshipit-source-id: 152554bc9864cdf25c569216d21b0c04bea67e9d

folly/portability/Constexpr.h

@@ -83,6 +83,7 @@ constexpr int constexpr_strcmp_internal(
   while (*s1 && *s1 == *s2) {
     ++s1, ++s2;
   }
+  // NOTE: `int(*s1 - *s2)` may cause signed arithmetics overflow which is UB.
   return int(*s2 < *s1) - int(*s1 < *s2);
 }