xo-interpreter2 stack: + reason arg to visit_gco_children()

Helps streamline DX1Collector in xo-gc/.
Want both forward and verify entry points for the same
representation.

src/reader2/DToplevelSeqSsm.cpp

@@ -518,7 +518,7 @@ namespace xo {
                  refrtag("seqtype", seqtype_));
         }
         void
-        DToplevelSeqSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DToplevelSeqSsm::visit_gco_children(VisitReason, obj<AGCObjectVisitor>) noexcept
         {
             // seqtype_: POD, skip
         }