Fix cpp syntax error when the service method name is `request`. (#18174)

Issue #18173

Closes #18174

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/18174 from liqipeng:main_fix_bug_method_name_is_request 502a4cdd44
PiperOrigin-RevId: 672951448

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

@@ -256,8 +256,10 @@ void ServiceGenerator::GenerateCallMethodCases(io::Printer* printer) {
         },
         R"cc(
           case $index$:
-            $name$(controller, ::$proto_ns$::DownCastMessage<$input$>(request),
-                   ::$proto_ns$::DownCastMessage<$output$>(response), done);
+            this->$name$(controller,
+                         ::$proto_ns$::DownCastMessage<$input$>(request),
+                         ::$proto_ns$::DownCastMessage<$output$>(response),
+                         done);
             break;
         )cc");
   }