Make C++ string oneof field compatible with C++11

string oneof field was generated into "const ArenaStringPtr" field inside
"default_oneof_instance_" struct (of name "<class name>OneofInstance").

On the other hand, in C++11, const field of type with trivial default
constructor causes enclosing type's implicit default constructor to be deleted.

Since ArenaStringPtr has tirvial default constructor, this caused default
constructor of "default_oneof_instance_" struct to be deleted, making the
constructor call inside generated code invalid and fail to compile.

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

@@ -1226,7 +1226,7 @@ GenerateDescriptorDeclarations(io::Printer* printer) {
       for (int j = 0; j < descriptor_->oneof_decl(i)->field_count(); j++) {
         const FieldDescriptor* field = descriptor_->oneof_decl(i)->field(j);
         printer->Print("  ");
-        if (IsStringOrMessage(field)) {
+        if (field->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
           printer->Print("const ");
         }
         field_generators_.get(field).GeneratePrivateMembers(printer);

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

@@ -613,8 +613,7 @@ GenerateSwappingCode(io::Printer* printer) const {
 void StringOneofFieldGenerator::
 GenerateConstructorCode(io::Printer* printer) const {
   printer->Print(variables_,
-    "  const_cast< ::google::protobuf::internal::ArenaStringPtr*>("
-    "&$classname$_default_oneof_instance_->$name$_)->UnsafeSetDefault("
+    "  $classname$_default_oneof_instance_->$name$_.UnsafeSetDefault("
     "$default_variable$);\n");
 }