AST passes assume that you are migrating from a version earlier
than Swift 4, where declaration references and type names may be
unconditionally renamed if their USRs match.
For example, this can happen for TypeMemberDiffItem entries where the
Objective-C USR is the same in Swift 3 and Swift 4, but the type is
spelled differently in Swift 4. A concrete example of this is:
`NSDocumentTypeDocumentAttribute` (Swift 3) ->
`NSAttributedString.DocumentAttributeKey` (Swift 4).
Although this declaration is imported differently in Swift 4, its
Objective-C USR is `c:@NSDocumentTypeDocumentAttribute` for both.
rdar://problem/32604558
