internal/core/adt: add missing dereference

This affects some OpenAPI tests.

Fixes #3847
Fixes #3825

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I570abc76e392546a45735bacb721cb99755cbda4
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1212215
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>

Author: mpvl

encoding/openapi/testdata/embed-norefs.txtar

@@ -1,5 +1,4 @@
 #ExpandReferences
-#skip-v3
 -- in.cue --
 #Foo: string
 

encoding/openapi/testdata/struct.txtar

@@ -1,4 +1,3 @@
-#skip-v3
 -- in.cue --
 import "struct"
 

encoding/openapi/testdata/structural.txtar

@@ -1,5 +1,4 @@
 #ExpandReferences
-#skip-v3
 -- in.cue --
 import "time"
 

internal/core/adt/composite.go

@@ -760,6 +760,7 @@ func (v *Vertex) IsData() bool {
 // of this vertex. Arcs are left untouched.
 // It is used by cue.Eval to convert nodes to data on per-node basis.
 func (v *Vertex) ToDataSingle() *Vertex {
+	v = v.DerefValue()
 	w := *v
 	w.isData = true
 	w.state = nil