Add a way for source languages to "mark" ValueObjects with language-specific flags

In this way, when a language needs to tell itself things that are not bound to a type but to a value (imagine a base-class relation, this is not about the type, but about the ValueObject), it can do so in a clean and general fashion

The interpretation of the values of the flags is, of course, up to the language that owns the value (the value object's runtime language, that is)

llvm-svn: 252503