SE-0025 has been accepted, and needs an implementation. This involves several pieces of work:
Introduce a new "fileprivate" declmodifier, which has the Swift 2.2 "private" semantics.
Rework "private" decls to only being visible within their scope, not within the current file.
Implement fixit/migration hint to indicate whether a "private" decl should be marked fileprivate.