Different from type hoist that moves global variables to static member
variables, we've also seen member variables being moved among different
types via apinotes. Swift-api-digester should be able to detect such
case so that migrator can handle them properly.
rdar://32466196
This item is to associate a usr with a special case Id, however, it's
up to the migrator to interpret the special case Id and apply proper
transformations.
This structure serves multiple purposes:
a. Allow migrator to speak in the same language with swift-api-digester in terms of API changes.
b. Serialize/Deserialize detected API change items in JSON format.
c. Manage memory after deserializing API change items.
d. Facilitate look up by USRs of APIs under migration.
The structure is tested by round-trip serialization and deserialization.