Handle %token variable references in Yacc grammars

A yacc grammar may have the following

```
%union {
  int integer;
}

%type <integer> rule
```

The `integer` is a field in the semantic stack, and the `%type` indicates that the listed rule "returns" that field.

`c-xrefactory` doesn't understand the `%type` declaration so a rename of the field will miss that occurrence.