Support changing the base type of a class to a child of the original

[sharwell]

The following transformation shouldn't be considered a breaking change, but the current implementation is "too strict" and treats all changes to the base type as a breaking change.

Before:

public class Foo
{
}

public class Bar
{
}

After:

public class Foo
{
}

public class Bar : Foo
{
}

[ArcEarth]

A stranger examples here:
This will not break APIL

API before change:

class Foo
{
    public virtual void Bar() {}
    public virtual void Baz() {}
}

API after change:

class FooBase
{
    public virtual void Bar() {}
}

class Foo : FooBase
{
    public virtual void Baz() {}
}

[sharwell]

Note that there is a subtle class of breaking changes here related to dotnet/csharplang#409, but it should still typically be allowed.