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/HashedCollections.swift.gyb

@@ -3319,7 +3319,7 @@ final internal class _Native${Self}StorageOwner<${TypeParametersDecl}>
   @objc
   internal required init(
     objects: UnsafePointer<AnyObject?>,
-    forKeys: UnsafeRawPointer,
+    forKeys: UnsafePointer<Void>,
     count: Int
   ) {
     _sanityCheckFailure("don't call this designated initializer")