swift234

[PrintAsObjC] Add unavailable attribute to unavailable obj-c initializers (#3852)

* [PrintAsObjC] Add unavailable attribute to non-inherited initializers

Initializers that aren't inherited by subclasses cannot be called, so we
should make this visible to Obj-C.

Due to SR-2211, non-inherited convenience initializers do not get this
same treatment.

* [PrintAsObjC] Add unavailable initializers for private overrides

When a public initializer is overridden with a private one, we need to
mark these as unavailable to Obj-C as they're not supposed to be
callable even though they do exist.