Consider mutating calls for free function stereotyping

Stereocode should consider calls on parameters passed by reference, globals, or static variables for free function stereotyping. For example:
```c++
void foo(object& bar) {
    bar.baz()
}
```

The call ```baz``` on ```bar``` should be considered as a mutator. We need something similar to ```findCallsOnDataMembers()``` but for the free functions.