[Demangling] Fix demangling of built generics to be more robust.

The type being rebuilt might not have any children.

rdar://91831405

Author: al45tair

lib/Demangling/OldDemangler.cpp

@@ -1044,7 +1044,8 @@ class OldDemangler {
       // had its generic arguments applied.
       NodePointer result = Factory.createNode(nominalType->getKind());
       result->addChild(parentOrModule, Factory);
-      result->addChild(nominalType->getChild(1), Factory);
+      for (unsigned ndx = 1; ndx < nominalType->getNumChildren(); ++ndx)
+        result->addChild(nominalType->getChild(ndx), Factory);
 
       nominalType = result;
     }