cue: Elem should return unevaluated value

This is tested in the openapi package

Change-Id: Idaadf085626de0d5a067b02d97b8da54510aacf0
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2347
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/testdata/oneof.cue b/encoding/openapi/testdata/oneof.cue
new file mode 100644
index 0000000..d318c50
--- /dev/null
+++ b/encoding/openapi/testdata/oneof.cue
@@ -0,0 +1,13 @@
+MyString: {
+	exact: string
+} | {
+	regex: string
+}
+
+MyInt: int
+
+Foo: {
+	include: MyString
+	exclude: [...MyString]
+	count: MyInt
+}