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/DExpectFormalArgSsm.cpp

@@ -265,7 +265,8 @@ namespace xo {
         }
 
         void
-        DExpectFormalArgSsm::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DExpectFormalArgSsm::visit_gco_children(VisitReason,
+                                                obj<AGCObjectVisitor>) noexcept
         {
             static_assert(!DUniqueString::is_gc_eligible());
         }