APICompat doesn't catch changing existing member to abstract

[ericstj]

Describe the bug

See discussion here dotnet/maintenance-packages#205 (comment)

To Reproduce

Make a package with a netstandard2.0 implementation with method not abstract. Have a compatible framewwork with the same member but abstract.

Expect: CannotAddAbstractMember diagnostic

Actual: No diagnostic. 

[dotnet-policy-service]

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