This handles optionality changes and type rewrites in function param and return types and constructor param and failability types.
Resolves rdar://problem/31766010
These data files are installed into runtime resource directory so that migrator can pick them automatically according to specific platforms. To support testing, a front-end option -api-diff-data-file can be used to specify the data file to use and it will overwrite the default ones from resource directory.
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.
