Rename ObjCMutablePointer to AutoreleasingUnsafePointer.
Rename ObjCMutablePointer to AutoreleasingUnsafePointer.

This relates its interface and behavior to that of UnsafePointer, and draws an analogy to '__autoreleasing *' in ARC.

Swift SVN r18236

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation