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

This reverts commit ece0951924.

This results in lldb failues on linux that I can't readily debug.
Backing out until they can be resolved.

stdlib/public/core/ShadowProtocols.swift

@@ -89,7 +89,7 @@ public protocol _NSDictionaryCore :
   // The designated initializer of `NSDictionary`.
   init(
     objects: UnsafePointer<AnyObject?>,
-    forKeys: UnsafeRawPointer, count: Int)
+    forKeys: UnsafePointer<Void>, count: Int)
 
   var count: Int { get }