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/SIL/Verifier.cpp

@@ -1386,6 +1386,7 @@ public:
     switch (kind) {
     case CheckedCastKind::Unresolved:
     case CheckedCastKind::Coercion:
+    case CheckedCastKind::ArrayDowncast:        
       llvm_unreachable("invalid for SIL");
     case CheckedCastKind::Downcast:
       require(fromTy.getClassOrBoundGenericClass(),