adjust config for FOLLY_SUPPLY_MISSING_INT128_TRAITS

Summary:
* remove the redundant `!defined(_WIN32)` condition;
* llvm added traits for (u)int128 in
  https://reviews.llvm.org/D2917; and std::hash in
  https://reviews.llvm.org/rL266587.

Reviewed By: nbronson

Differential Revision: D7111138

fbshipit-source-id: 91a05354846575677eddd48c8b8f4e05c53b01cb

CMake/FollyConfigChecks.cmake

@@ -123,11 +123,16 @@ check_type_size(__int128 INT128_SIZE LANGUAGE CXX)
 if (NOT INT128_SIZE STREQUAL "")
   set(FOLLY_HAVE_INT128_T ON)
   check_cxx_source_compiles("
+    #include <functional>
     #include <type_traits>
+    #include <utility>
     static_assert(
       ::std::is_same<::std::make_signed<unsigned __int128>::type,
                      __int128>::value,
       \"signed form of 'unsigned __uint128' must be '__int128'.\");
+    static_assert(
+        sizeof(::std::hash<__int128>{}(0)) > 0, \"std::hash<__int128> is
+        disabled.\");
     int main() { return 0; }"
     HAVE_INT128_TRAITS
   )