More array casting work:

- Continue adding support for checked downcasts of array types (rdar://problem/16535104)
- Fix non-bridged array conversions post-r17868
- Fix rdar://problem/16773693
- Add tests for NSArray coercions to and from Array<T>

Swift SVN r17957

lib/AST/ASTPrinter.cpp

@@ -2168,6 +2168,8 @@ StringRef swift::getCheckedCastKindName(CheckedCastKind kind) {
     return "concrete_to_archetype";
   case CheckedCastKind::ConcreteToUnrelatedExistential:
     return "concrete_to_unrelated_existential";
+  case CheckedCastKind::ArrayDowncast:
+    return "array_downcast";
   }
   llvm_unreachable("bad checked cast name");
 }