v2.6.0: Add support for invokable classes as macro function (#765)

Macros can be used with invokable classes, like so.

```
<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use Illuminate\Support\Facades\Response;

class ResponseMacroServiceProvider extends ServiceProvider
{
    public function boot()
    {
        Response::macro('foo', new Foo());
    }
}

class Foo
{
    public function __invoke()
    {
        return 'foobar';
    }
}
```

When running `ide-helper:generate` the following fatal error was thrown.

```  Symfony\Component\Debug\Exception\FatalThrowableError  : ReflectionFunction::__construct() expects parameter 1 to be string, object given```

This commit fixes this error.