[Conv] MaxString specialization for __int128

Summary: Just that.

Test Plan: .

Reviewed By: igorzi@fb.com

FB internal diff: D613291

folly/Conv.cpp

@@ -42,6 +42,8 @@ static_assert(sizeof(unsigned long) >= 4,
               " please update.");
 template <> const char *const MaxString<unsigned long long>::value =
   "18446744073709551615";
+template <> const char *const MaxString<unsigned __int128>::value =
+  "340282366920938463463374607431768211455";
 static_assert(sizeof(unsigned long long) >= 8,
               "Wrong value for MaxString<unsigned long long>::value"
               ", please update.");