Add missing < 0 tests for constexpr_strcmp

Summary: [Folly] Add missing `< 0` tests for `constexpr_strcmp` to verify that the implementation is symmetric.

Reviewed By: Mizuchi

Differential Revision: D23482134

fbshipit-source-id: d3cb1fe8ce4690a34a12620eb57e7a8ff57ad8a1

folly/portability/test/ConstexprTest.cpp

@@ -60,9 +60,12 @@ TEST(ConstexprTest, constexpr_strcmp_ints) {
   constexpr int v[] = {5, 3, 4, 0, 7};
   constexpr int v1[] = {6, 4};
   static_assert(constexpr_strcmp(v1, v) > 0, "constexpr_strcmp is broken");
+  static_assert(constexpr_strcmp(v, v1) < 0, "constexpr_strcmp is broken");
   static_assert(constexpr_strcmp(v, v) == 0, "constexpr_strcmp is broken");
   static_assert(
       constexpr_strcmp_fallback(v1, v) > 0, "constexpr_strcmp is broken");
+  static_assert(
+      constexpr_strcmp_fallback(v, v1) < 0, "constexpr_strcmp is broken");
   static_assert(
       constexpr_strcmp_fallback(v, v) == 0, "constexpr_strcmp is broken");
 }