C++ interop: generated header: invalid header generated when forward declaration is required between two classes

### Description

THe generated header doesn't generate forward declarations between two Swift classes:

### Reproduction

```swift
public class B
{
    public init() {}
}

public class A
{
    public init(_ b: B) {}
}
```


### Expected behavior

The generated header should be valid.

### Environment

Swift main

### Additional information

_No response_