cue: fix export bug

Everything worked fine when formatting the
export and then parsing back, but this broke
when exported AST was used directly.

Change-Id: I4281735b7a68900a6cfd3d3cfc34cbb2c3d08ada
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5085
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cue/export.go b/cue/export.go
index 9abf05c..5ab732f 100644
--- a/cue/export.go
+++ b/cue/export.go
@@ -534,10 +534,10 @@
 		}
 
 	case *nullLit:
-		return p.ident("null")
+		return &ast.BasicLit{Kind: token.NULL, Value: "null"}
 
 	case *boolLit:
-		return p.ident(fmt.Sprint(x.b))
+		return ast.NewBool(x.b)
 
 	case *stringLit:
 		return &ast.BasicLit{