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.

validation-test/stdlib/ArrayNew.swift.gyb

@@ -20,19 +20,19 @@ all_array_types = ['ContiguousArray', 'ArraySlice', 'Array']
 }%
 
 extension Array {
-  var identity: UnsafeRawPointer {
+  var identity: UnsafePointer<Void> {
     return self._buffer.identity
   }
 }
 
 extension ArraySlice {
-  var identity: UnsafeRawPointer {
+  var identity: UnsafePointer<Void> {
     return self._buffer.identity
   }
 }
 
 extension ContiguousArray {
-  var identity: UnsafeRawPointer {
+  var identity: UnsafePointer<Void> {
     return self._buffer.identity
   }
 }