Introduce GenerateIfDirectiveAttribute

[mayuki]

namespace TempProject
{
    [GenerateIfDirective("MYDEBUG || DEBUG")]
    public interface IMyService : IService<IMyService>
    {
        UnaryResult<Nil> A(MyDebugObject a, MyDebugObject b);
    }

    [MessagePackObject]
    public class MyDebugObject
    {
    }
}