Casting runtime: Use the runtime's constant for reserved bits in an ObjC pointer.

Previously, we had a predicate that only worked properly for 64-bit
architectures; the 32-bit platforms don't have tagged pointers, and
have an annoying tendency to allocate objects with the high bit of the
address set. Fixes <rdar://problem/17544277>.

Swift SVN r19523

stdlib/runtime/Casting.cpp

@@ -87,7 +87,8 @@ uint8_t swift::swift_classShift = 0;
 #if SWIFT_OBJC_INTEROP
 /// Does this object use a tagged-pointer representation?
 static bool isTaggedPointerOrNull(const void *object) {
-  return ((long)object & 1) || ((long)object <= 0);
+  return ((uintptr_t)object & heap_object_abi::ObjCReservedBitsMask) || 
+         object == nullptr;
 }
 #endif