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/encoding/openapi/build.go b/encoding/openapi/build.go
index 8d690d1..5fee0a5 100644
--- a/encoding/openapi/build.go
+++ b/encoding/openapi/build.go
@@ -498,7 +498,7 @@
 		return
 	}
 
-	switch v.IncompleteKind() &^ cue.BottomKind {
+	switch v.IncompleteKind() {
 	case cue.BoolKind:
 		b.typ = "boolean"
 	case cue.FloatKind, cue.NumberKind:
@@ -522,7 +522,7 @@
 		return
 	}
 
-	switch v.IncompleteKind() &^ cue.BottomKind {
+	switch v.IncompleteKind() {
 	case cue.NullKind:
 		// TODO: for JSON schema we would set the type here. For OpenAPI,
 		// it must be nullable.