[stdlib] Kill off _isUniquelyReferenced

It was doing an unsafeBitCast and possibly not managing lifetimes;
replace it with more-typesafe and memory-safe calls where possible.

Swift SVN r22779

stdlib/core/SliceBuffer.swift

@@ -237,7 +237,7 @@ struct _SliceBuffer<T> : _ArrayBufferType {
   }
 
   mutating func isUniquelyReferenced() -> Bool {
-    return Swift._isUniquelyReferenced(&owner)
+    return isUniquelyReferencedNonObjC(&owner)
   }
 
   /// Access the element at `position`.