Inline yield in Generator complains about missing return type

PHPStan reports a false positive when inline yield is used:

```php
<?php

function foo(): Generator
{
    while ($bar = yield 'foo') {
    }
}
```

```
Function foo() should return Generator but return statement is missing.
```

https://phpstan.org/r/ec37ffcb-25f5-424a-b21c-2f5ec10703dd