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.