In the IsStructurallyValidLanguageTag AO, we of course cannot assume the
variants are canonicalized to lower-case yet, because canonicalization
hasn't happened yet.
ErrorType::IntlInvalidCode has almost exactly the same message as
ErrorType::OptionIsNotValidValue. Remove it, as all uses of the former
are semantically interchangeable with the latter.
Note that only option type=region is really implemented. Other types
will resort to the fallback option. This prototype method will be able
to implement other type options once LibUnicode supports more.
There is notably FIXME notations in this commit regarding Unicode locale
extensions. We are not parsing extensions (or private use extensions) at
all yet.
