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/DExpectSymbolSsm.cpp

@@ -146,7 +146,8 @@ namespace xo {
                     );
         }
         void
-        DExpectSymbolSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectSymbolSsm::visit_gco_children(VisitReason,
+                                             obj<AGCObjectVisitor>) noexcept
         {
             // no gc-aware members
         }