* [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
