Handle LazyRefs conservatively in isReferredTo

Without the change we risk losing RecTypes because
it looks like nobody refers to them. This was observed
for pos/i974.scala.

Author: odersky

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

@@ -2265,6 +2265,8 @@ object Types {
           tp match {
             case tp: TypeRef => apply(x, tp.prefix)
             case tp: RecThis => RecType.this eq tp.binder
+            case tp: LazyRef => true // Assume a reference to be safe.
+              // TODO: Check that all accumulators handle LazyRefs correctly
             case _ => foldOver(x, tp)
           }
         }