OnRefFuncExpr takes a func index. NFC (#1768)
The name of this argument was recently changed from func_index
to type_index, but I think that might have been incorrect.
The immediate that read in the binary reader is (IIRC) a function
index:
```
case Opcode::RefFunc: {
Index func;
CHECK_RESULT(ReadIndex(&func, "func index"));
CALLBACK(OnRefFuncExpr, func);
CALLBACK(OnOpcodeUint32, func);
break;
}
```
and not a type index. Indeed the interpreter seems to treat it
as a function index too:
```
Result BinaryReaderInterp::OnRefFuncExpr(Index func_index) {
CHECK_RESULT(validator_.OnRefFunc(loc, Var(func_index)));
istream_.Emit(Opcode::RefFunc, func_index);
return Result::Ok;
}
```
