swift234

Fix two problems with implicit conversions:
- rdar://problem/16776273, wherein conversions between nil and .None were permitted
due to an implicit conversion between nil and COpaquePointer.
- rdar://problem/16877526, where we needed to add new equality overloads to handle
conversions between nil and .None given the supression of user conversions.
(Thanks to Ted for the overloads and test.)

Swift SVN r18473