ARROW-10490: [C++][GLib] Fix range-loop-analysis warnings

They are detected by Xcode 12.

Closes #8587 from kou/xcode-12

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

c_glib/arrow-glib/reader.cpp

@@ -1184,7 +1184,7 @@ garrow_csv_read_options_add_schema(GArrowCSVReadOptions *options,
 {
   auto priv = GARROW_CSV_READ_OPTIONS_GET_PRIVATE(options);
   auto arrow_schema = garrow_schema_get_raw(schema);
-  for (const auto field : arrow_schema->fields()) {
+  for (const auto &field : arrow_schema->fields()) {
     priv->convert_options.column_types[field->name()] = field->type();
   }
 }
@@ -1206,7 +1206,7 @@ garrow_csv_read_options_get_column_types(GArrowCSVReadOptions *options)
                                             g_str_equal,
                                             g_free,
                                             g_object_unref);
-  for (const auto iter : priv->convert_options.column_types) {
+  for (const auto &iter : priv->convert_options.column_types) {
     auto arrow_name = iter.first;
     auto arrow_data_type = iter.second;
     g_hash_table_insert(types,

cpp/src/plasma/protocol.cc

@@ -581,7 +581,7 @@ Status ReadListReply(const uint8_t* data, size_t size, ObjectTable* objects) {
   DCHECK(data);
   auto message = flatbuffers::GetRoot<fb::PlasmaListReply>(data);
   DCHECK(VerifyFlatbuffer(message, data, size));
-  for (auto const& object : *message->objects()) {
+  for (auto const object : *message->objects()) {
     ObjectID object_id = ObjectID::from_binary(object->object_id()->str());
     auto entry = std::unique_ptr<ObjectTableEntry>(new ObjectTableEntry());
     entry->data_size = object->data_size();