[APINotes] Preserve attributes from inactive versions. (#53)
...by wrapping them in another (new) attribute, SwiftVersionedAttr,
or by noting their deletion in a SwiftVersionedRemovalAttr.
This doesn't support other kinds of changes that can be made via API
notes, such as nullability or wholesale type changes, but it's a place
to start, and possibly sufficient for our goals.
Part of rdar://problem/28618121.