Sema: NSValue-to-value-type casts are failable and should be checked.

In Swift 4 mode, no longer consider e.g. 'nsNumber as Int' or 'nsValue as NSRange' to be valid coercions. This would break compatibility with Swift 3, so in Swift 3 mode, accept the coercion, but *also* accept a checked cast without a warning, and raise a migration warning about the unchecked coercion.

lib/AST/ASTPrinter.cpp

@@ -4114,8 +4114,10 @@ StringRef swift::getCheckedCastKindName(CheckedCastKind kind) {
     return "dictionary_downcast";
   case CheckedCastKind::SetDowncast:
     return "set_downcast";
-  case CheckedCastKind::BridgingCast:
-    return "bridging_cast";
+  case CheckedCastKind::BridgingCoercion:
+    return "bridging_coercion";
+  case CheckedCastKind::Swift3BridgingDowncast:
+    return "bridging_downcast";
   }
   llvm_unreachable("bad checked cast name");
 }