Enable indexing for refs to synthesized declarations.
Based on feedback in PR https://github.com/swiftlang/swift/pull/69460, enabling indexing for synthesized decls because they are usable by users and make sense to appear in the indexstore.
Sets `synthesized` on some additional decls:
- derived `hashInto(...)`
- Objc properties and methods derived from Objc protocols
https://github.com/apple/swift/issues/67446
