APICompat should warn for nullable-enabled implementation of nullable-disabled base member

[sharwell]

Describe the bug

ApiCompat fails to warn users when they expose a nullable-enabled implementation of a member which is not nullable-enabled. This can lead to errors in the future if the base member becomes null-annotated in a way which is not compatible with the implementing signature.

Special cases to consider:

1. If the return type of the implementation is declared non-nullable, there may be no need to create a warning. Even if the base type is updated to return a nullable type, the implementation is allowed to further constrain the value to non-null.
   • Note that if the implementation includes a line that returns the result of calling the base member, a change to the base signature might necessitate a change to the implementation signature, so the warning might still be valuable.
2. If the type of a parameter is declared nullable, there may be no need to create a warning. Even if the base type is updated to only accept a non-nullable type, the implementation is allowed to further relax the value to nullable.
   • Note that if the implementation includes a call to the base member and passes the parameter value, a change to the base signature might necessitate a change to the implementation signature, so the warning might still be valuable.

To Reproduce

#nullable disable

internal class BaseType
{
    protected virtual void Method(object obj) { }
}

#nullable enable

internal class DerivedType : BaseType
{
    protected override void Method(object obj) { }
}

[ghost]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage

[ericstj]

This should be covered as part of #18681