swift234

[stdlib] Revert UnsafePointer casting change

The syntax being reverted added busywork and noise to the common case
where you want to say "I have the right address, but the wrong type,"
without adding any real safety.

Also it eliminated the ability to write UnsafePointer<T>(otherPointer),
without adding ".self" to T.  Overall, it was not a win.

This reverts commits r21324 and r21342

Swift SVN r21424