Breaking Change: Ensure we allocate overaligned types with aligned operator new.

* The alignment may be incorrect.
* Using delete[] otherwise mismatches, since it was not allocated with
  overaligned operator new.

Additionally, we add a test to ensure we do not value initialize in this case.

PiperOrigin-RevId: 620241337

src/google/protobuf/arena.h

@@ -266,7 +266,7 @@ class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final {
     ABSL_CHECK_LE(num_elements, std::numeric_limits<size_t>::max() / sizeof(T))
         << "Requested size is too large to fit into size_t.";
     if (PROTOBUF_PREDICT_FALSE(arena == nullptr)) {
-      return static_cast<T*>(::operator new[](num_elements * sizeof(T)));
+      return new T[num_elements];
     } else {
       // We count on compiler to realize that if sizeof(T) is a multiple of
       // 8 AlignUpTo can be elided.