[obj-c] Drop remaining usages of 'using std::string' (#31517)

src/compiler/objective_c_generator.h

@@ -30,30 +30,29 @@ struct Parameters {
 
 using ::grpc::protobuf::FileDescriptor;
 using ::grpc::protobuf::ServiceDescriptor;
-using ::std::string;
 
 // Returns forward declaration of classes in the generated header file.
-string GetAllMessageClasses(const FileDescriptor* file);
+std::string GetAllMessageClasses(const FileDescriptor* file);
 
 // Returns the content to be included defining the @protocol segment at the
 // insertion point of the generated implementation file. This interface is
 // legacy and for backwards compatibility.
-string GetProtocol(const ServiceDescriptor* service,
-                   const Parameters& generator_params);
+std::string GetProtocol(const ServiceDescriptor* service,
+                        const Parameters& generator_params);
 
 // Returns the content to be included defining the @protocol segment at the
 // insertion point of the generated implementation file.
-string GetV2Protocol(const ServiceDescriptor* service);
+std::string GetV2Protocol(const ServiceDescriptor* service);
 
 // Returns the content to be included defining the @interface segment at the
 // insertion point of the generated implementation file.
-string GetInterface(const ServiceDescriptor* service,
-                    const Parameters& generator_params);
+std::string GetInterface(const ServiceDescriptor* service,
+                         const Parameters& generator_params);
 
 // Returns the content to be included in the "global_scope" insertion point of
 // the generated implementation file.
-string GetSource(const ServiceDescriptor* service,
-                 const Parameters& generator_params);
+std::string GetSource(const ServiceDescriptor* service,
+                      const Parameters& generator_params);
 
 }  // namespace grpc_objective_c_generator
 

src/compiler/objective_c_generator_helpers.h

@@ -31,9 +31,8 @@ namespace grpc_objective_c_generator {
 using ::grpc::protobuf::FileDescriptor;
 using ::grpc::protobuf::MethodDescriptor;
 using ::grpc::protobuf::ServiceDescriptor;
-using ::std::string;
 
-inline string MessageHeaderName(const FileDescriptor* file) {
+inline std::string MessageHeaderName(const FileDescriptor* file) {
   return google::protobuf::compiler::objectivec::FilePath(file) + ".pbobjc.h";
 }