swift234

Disallow illegal UnsafePointer conversions. (#3858)

As proposed in SE-0107: UnsafeRawPointer.
https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md

Remove the initializers that allow type inference to
change the Pointee type of an UnsafePointer.
These initializers violate the strict aliasing
aspect of the memory model and can lead to undefined
behavior.