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-stringtable2/src/stringtable2/DUniqueString.cpp

@@ -109,7 +109,7 @@ namespace xo {
         }
 
         void
-        DUniqueString::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DUniqueString::visit_gco_children(VisitReason, obj<AGCObjectVisitor>) noexcept
         {
             // no-op -- childless!
         }