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/ASTDumper.cpp

@@ -1364,6 +1364,11 @@ public:
     printRec(E->getSubExpr());
     OS << ')';
   }
+  void visitArrayDowncastConversionExpr(ArrayDowncastConversionExpr *E) {
+    printCommon(E, "array_downcast_expr") << '\n';
+    printRec(E->getSubExpr());
+    OS << ')';
+  }
   void visitArrayBridgedConversionExpr(ArrayBridgedConversionExpr *E) {
     printCommon(E, "array_bridged_conversion_expr") << '\n';
     printRec(E->getSubExpr());