Enable small object optimization in swisstable.

See [implementation commit](https://github.com/abseil/abseil-cpp/commit/1449c9a106b090f61441ba245c781d7d2f89000c) for design details.

PiperOrigin-RevId: 619309882
Change-Id: I093c00365dda2268be86ba3d21421b6ffb59a5ce

absl/container/internal/hash_policy_traits.h

@@ -168,7 +168,7 @@ struct hash_policy_traits : common_policy_traits<Policy> {
 #endif
 #endif
   }
   }
 
 
-  // Whether small object optimization is enabled. False by default.
+  // Whether small object optimization is enabled. True by default.
   static constexpr bool soo_enabled() { return soo_enabled_impl(Rank1{}); }
   static constexpr bool soo_enabled() { return soo_enabled_impl(Rank1{}); }
 
 
  private:
  private:
@@ -197,7 +197,7 @@ struct hash_policy_traits : common_policy_traits<Policy> {
     return P::soo_enabled();
     return P::soo_enabled();
   }
   }
 
 
-  static constexpr bool soo_enabled_impl(Rank0) { return false; }
+  static constexpr bool soo_enabled_impl(Rank0) { return true; }
 };
 };
 
 
 }  // namespace container_internal
 }  // namespace container_internal