* [InterfaceGen] Print abstract accessors in protocols
This patch slightly cleans up printing accessors and ensures we print
accessors abstractly in protocol context for textual interfaces.
It also removes some assuptions around the FunctionBody callback and
makes them more explicit.
* Print getter and setter for didSet decls
* Test _read and _modify
* Fix logic for skipping willSet/didSet
* Update 'final' test for new getter printing behavior
* Introduce stored inlinable function bodies
* Remove serialization changes
* [InterfaceGen] Print inlinable function bodies
* Clean up a little bit and add test
* Undo changes to InlinableText
* Add serialization and deserialization for inlinable body text
* Allow parser to parse accessor bodies in interfaces
* Fix some tests
* Fix remaining tests
* Add tests for usableFromInline decls
* Add comments
* Clean up function body printing throughout
* Add tests for subscripts
* Remove comment about subscript inlinable text
* Address some comments
* Handle lack of @objc on Linux
