explicitly qualified as members of the current instantiation.
Despite the nested name specifier being fully-dependent in this case,
the elaborated type might only be instantiation-dependent, because the
type is a member of the current instantiation.
the nested-name-specifier when determining whether a qualified type is
instantiation-dependent.
Previously reverted in 25a02c3d1a688d3cd18faef96c75fa553efbbac7 due to
causing us to reject some code. It turns out that the rejected code was
ill-formed (no diagnostic required).
