Migrate from UnsafePointer<Void> to UnsafeRawPointer. (#3724)

* Migrate from `UnsafePointer<Void>` to `UnsafeRawPointer`.

As proposed in SE-0107: UnsafeRawPointer.

`void*` imports as `UnsafeMutableRawPointer`.
`const void*` imports as `UnsafeRawPointer`.

Occurrences of `UnsafePointer<Void>` are replaced with UnsafeRawPointer.

* Migrate overlays from UnsafePointer<Void> to UnsafeRawPointer.

This requires explicit memory binding in several places,
particularly in NSData and CoreAudio.

* Fix a bunch of test cases for Void->Raw migration.

* qsort takes IUO values

* Bridge `Unsafe[Mutable]RawPointer as `void [const] *`.

* Parse #dsohandle as UnsafeMutableRawPointer

* Update a bunch of test cases for Void->Raw migration.

* Trivial fix for the SceneKit test case.

* Add an UnsafeRawPointer self initializer.

This is unfortunately necessary for assignment between types imported from C.

* Tiny simplification of the initializer.

stdlib/public/core/ArrayBufferProtocol.swift

@@ -120,7 +120,7 @@ public protocol _ArrayBufferProtocol
   /// A value that identifies the storage used by the buffer.  Two
   /// buffers address the same elements when they have the same
   /// identity and count.
-  var identity: UnsafePointer<Void> { get }
+  var identity: UnsafeRawPointer { get }
 
   var startIndex: Int { get }
 }