[Concurrency] Fix ObjC weak references to actors.
Change actor destruction to call swift_deallocClassInstance instead of swift_deallocObject. When ObjC interop is enabled, swift_deallocClassInstance will check the "pure swift deallocation" bit and call into the ObjC runtime to destruct the instance when needed. This is what clears weak references and associated objects.
rdar://91270492
(cherry picked from commit b00397c7ebc89bbdd8e0eb30ea056400c10542b6)
