Change LazyField to use a flat char array when the unparsed payload i…

…s small. This array can easily be allocated in the arena avoiding separate heap usage of the Cord.

Also, we remove the ArenaDtor and register the Cord for destruction on demand.
This avoids registering the ArenaDtor on every message with a lazy field.

PiperOrigin-RevId: 620857434

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

@@ -1473,7 +1473,8 @@ void MessageGenerator::GenerateImplDefinition(io::Printer* p) {
           inline explicit Impl_($pbi$::InternalVisibility visibility,
                                 ::$proto_ns$::Arena* arena);
           inline explicit Impl_($pbi$::InternalVisibility visibility,
-                                ::$proto_ns$::Arena* arena, const Impl_& from);
+                                ::$proto_ns$::Arena* arena, const Impl_& from,
+                                const $classname$& from_msg);
           //~ Members assumed to align to 8 bytes:
           $extension_set$;
           $tracker$;
@@ -2993,7 +2994,7 @@ void MessageGenerator::GenerateArenaEnabledCopyConstructor(io::Printer* p) {
         R"cc(
           inline PROTOBUF_NDEBUG_INLINE $classname$::Impl_::Impl_(
               $pbi$::InternalVisibility visibility, ::$proto_ns$::Arena* arena,
-              const Impl_& from)
+              const Impl_& from, const $classtype$& from_msg)
               //~
               $init$ {}
         )cc");
@@ -3003,7 +3004,7 @@ void MessageGenerator::GenerateArenaEnabledCopyConstructor(io::Printer* p) {
   auto copy_construct_impl = [&] {
     if (!HasSimpleBaseClass(descriptor_, options_)) {
       p->Emit(R"cc(
-        new (&_impl_) Impl_(internal_visibility(), arena, from._impl_);
+        new (&_impl_) Impl_(internal_visibility(), arena, from._impl_, from);
       )cc");
     }
   };