Replace is_trivially_default_constructible with is_trivial for old libstdc++.

src/google/protobuf/arena.h

@@ -327,7 +327,7 @@ class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final {
   template <typename T>
   PROTOBUF_NDEBUG_INLINE static T* CreateArray(Arena* arena,
                                                size_t num_elements) {
-    static_assert(std::is_trivially_default_constructible<T>::value,
+    static_assert(std::is_trivial<T>::value,
                   "CreateArray requires a trivially constructible type");
     static_assert(std::is_trivially_destructible<T>::value,
                   "CreateArray requires a trivially destructible type");