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.

xo-reader2/src/reader2/DExpectExprSsm.cpp

@@ -625,7 +625,8 @@ namespace xo {
 #endif
 
         void
-        DExpectExprSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectExprSsm::visit_gco_children(VisitReason,
+                                           obj<AGCObjectVisitor>) noexcept
         {
             // all members POD, skip
         }