cue: define BottomKind as 0

This simplifies APIs and confusion.

Change-Id: I0d780c2041e36d5b91fb361f1f2a06a071e7c36e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2955
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/pkg/tool/file/file.go b/pkg/tool/file/file.go
index b64a64d..694f832 100644
--- a/pkg/tool/file/file.go
+++ b/pkg/tool/file/file.go
@@ -54,7 +54,7 @@
 	}
 	update := map[string]interface{}{"contents": b}
 
-	switch v.Lookup("contents").IncompleteKind() &^ cue.BottomKind {
+	switch v.Lookup("contents").IncompleteKind() {
 	case cue.BytesKind:
 	case cue.StringKind:
 		update["contents"] = string(b)