When we reference a member of a class extension, we need to
compute its overrides, @objc bit, and ‘dynamic’ status because SILGen
may rely on them. Do that consistently by adding such members to the
list of declarations to “finalize”.
Rework the finalization logic to handle the computation of each of those
bits, and never remove a finalized declaration from the set: rather,
process new declarations as they enter the set, to avoid looping or
extra state bits.
Fixes rdar://problem/42440686.
