Bincompat hooks for revised hash/isEqual interop
This adds in hooks so that the new hash/isEqual interop
(which bridges Obj-C hash/isEqual: calls to the corresponding
Swift Hashable/Equatable conformances) can be selectively
disabled based on the OS and/or client.
For now, enable the new semantics everywhere except Apple platforms
(which have legacy apps that may be relying on the old semantics).
