[Issue 23240] dmd compiles 'ok' class invalid to interface if class contains abstract members

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23240

--- Comment #1 from Alexey <animuspexus at protonmail.com> ---
also, the following modification to the code compiles ok and resulting program
segfaults on run

```
import std.stdio;

interface ClassInterface
{
    ClassInterface setValue(string val);
}

class ClassPrototype : ClassInterface
{
    string value;

    void setValue(string val)
    {
        value = val;
    }

    abstract void doSomething();
}

class Class : ClassPrototype
{
    override void doSomething()
    {
        writeln("321");
    }
}

void main()
{
    auto inst = new Class();
    inst.setValue("123");
    ClassInterface zzz = cast(ClassInterface) inst;
    assert(zzz);
    auto xxx = zzz.setValue("333");
}
```

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