Use a pointer instead of a ref from C++ thunk.

While in practice a C++ ref and ptr will have the same ABI, since C doesn't have refs this gets flagged by -Wreturn-type-c-linkage. Note that this contrasts with the Rust side where C-ABI functions that take a pointer or ref are permitted and guaranteed to have the same ABI.

PiperOrigin-RevId: 676875775

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

@@ -1399,10 +1399,10 @@ void GenerateThunksCc(Context& ctx, const Descriptor& msg) {
         size_t $repeated_len_thunk$(google::protobuf::RepeatedPtrField<$QualifiedMsg$>* field) {
           return field->size();
         }
-        const $QualifiedMsg$& $repeated_get_thunk$(
+        const $QualifiedMsg$* $repeated_get_thunk$(
           google::protobuf::RepeatedPtrField<$QualifiedMsg$>* field,
           size_t index) {
-          return field->Get(index);
+          return &field->Get(index);
         }
         $QualifiedMsg$* $repeated_get_mut_thunk$(
           google::protobuf::RepeatedPtrField<$QualifiedMsg$>* field,