Breaking change: Add a debug check that the target of CopyFrom is not a descendant of the source.

Previously, you were allowed to do the following:
```cc
MyMessage msg;
*msg.mutable_submessage() = msg;
```

However, this could leave the message in an inconsistent state in some contrived examples, and the order of operations is unclear and potentially compiler-dependent.

Instead, if you would like to do this, you should be explicit about the order of operations:
```cc
MyMessage msg;
MyMessage msg_copy = msg;
*msg.mutable_submessage() = std::move(msg_copy);
```

PiperOrigin-RevId: 820462156

Author: ClaytonKnittel

src/google/protobuf/compiler/cpp/message.cc

@@ -4702,11 +4702,9 @@ void MessageGenerator::GenerateCopyFrom(io::Printer* p) {
                 $DCHK$(!::_pbi::IsDescendant(*this, from))
                     << "Source of CopyFrom cannot be a descendant of the "
                        "target.";
-#ifdef PROTOBUF_FUTURE_NO_RECURSIVE_MESSAGE_COPY
                 $DCHK$(!::_pbi::IsDescendant(from, *this))
                     << "Target of CopyFrom cannot be a descendant of the "
                        "source.";
-#endif  // PROTOBUF_FUTURE_NO_RECURSIVE_MESSAGE_COPY
                 Clear();)cc");
             } else {
               p->Emit(R"cc(

src/google/protobuf/message.cc

@@ -108,10 +108,8 @@ void Message::CopyFrom(const Message& from) {
     // Fail if "from" is a descendant of "to" as such copy is not allowed.
     ABSL_DCHECK(!internal::IsDescendant(*this, from))
         << "Source of CopyFrom cannot be a descendant of the target.";
-#ifdef PROTOBUF_FUTURE_NO_RECURSIVE_MESSAGE_COPY
     ABSL_DCHECK(!internal::IsDescendant(from, *this))
         << "Target of CopyFrom cannot be a descendant of the source.";
-#endif  // PROTOBUF_FUTURE_NO_RECURSIVE_MESSAGE_COPY
     Clear();
     class_to->full().merge_to_from(*this, from);
   } else {

src/google/protobuf/port_def.inc

@@ -157,10 +157,6 @@ static_assert(PROTOBUF_ABSL_MIN(20230125, 3),
 // Owner: ckennelly@, mkruskal@
 #define PROTOBUF_FUTURE_ADD_NODISCARD [[nodiscard]]
 
-// Removes the ability to copy messages into themselves.
-// Owner: cknittel@, mkruskal@
-#define PROTOBUF_FUTURE_NO_RECURSIVE_MESSAGE_COPY 1
-
 #else
 
 #define PROTOBUF_FUTURE_ADD_NODISCARD

src/google/protobuf/port_undef.inc

@@ -76,7 +76,6 @@
 #undef PROTOBUF_FUTURE_DESCRIPTOR_EXTENSION_DECL
 #undef PROTOBUF_FUTURE_REMOVE_CREATEMESSAGE
 #undef PROTOBUF_FUTURE_ADD_NODISCARD
-#undef PROTOBUF_FUTURE_NO_RECURSIVE_MESSAGE_COPY
 #endif
 
 #include "google/protobuf/os_macros_restore.inc"