Improve our handling of the current instantiation for qualified

id-expression, e.g., 

  CurrentClass<T>::member

Previously, if CurrentClass<T> was dependent and not complete, we
would treat it as a dependent-scoped declaration reference expression,
even if CurrentClass<T> referred to the current instantiation.

Fixes PR8966 and improves type checking of templates.

llvm-svn: 124867