[APINotes] Record what version caused an annotation to get replaced. (#123)
Now that -fapinotes-swift-version=3 can pick up API notes for
"Version: 4", the information for /inactive/ API notes isn't
sufficient to tell what would have happened under a different version.
That is, I can't ask "what would happen under version 4", because I
can't tell if the active annotations are active because they were
specified for version 3 or version 4. The inactive, unversioned
attributes were no help, either: they just said "version 0".
Fix this by adding a new flag 'IsReplacedByActive' to
SwiftVersionedAttr and SwiftVersionedRemovalAttr. When set, the
'Version' field refers to the API note that caused this annotation to
Note that "version 0" can still exist: when an attribute written in
source is replaced by an unversioned attribute in the API notes.