Update Int/UInt to use target machine pointer size

[Snowy1803]

Int and UInt should be equivalent to C ptrdiff_t and size_t, or Rust isize and usize, or Swift Int or UInt.
They should not use size 32 or 64, but the target machine's pointer size. This pointer size should be passed down from the Driver, maybe through the ImportManager that should have the target machine/triple info.