Make LazyRef#ref a def instead of a lazy val

It's slightly more efficient. There was no need to have
a separate cache for the lazy val.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -1972,14 +1972,15 @@ object Types {
   case class LazyRef(refFn: () => Type) extends UncachedProxyType with ValueType {
     private var myRef: Type = null
     private var computed = false
-    lazy val ref = {
+    def ref = {
       if (computed) assert(myRef != null)
       else {
         computed = true
         myRef = refFn()
       }
       myRef
     }
+    def evaluating = computed && myRef == null
     override def underlying(implicit ctx: Context) = ref
     override def toString = s"LazyRef($ref)"
     override def equals(other: Any) = other match {