@Override
    public void visit(Placeholder placeholder) {
        MapChoice choice = placeholderIdToChoice.get(placeholder.getId());
        if (choice == null)
            throw new IllegalArgumentException(placeholder + " is not referenced by any choice");
        placeholder.setValueContainer(choice);
    }
Creates references from all placeholders to the choices which resolves them.
If a placeholder is encountered which is not resolved by any choice, an IllegalArgumentException is thrown.

@author bratseth
container-search/src/main/java/com/yahoo/search/pagetemplates/PlaceholderReferenceCreatingVisitor.java