Can polymorphism help to group LinkageProblems?

(followup of https://github.com/GoogleCloudPlatform/cloud-opensource-java/pull/1604#discussion_r475624772 )

When printing `LinkageProblem`s, grouping them by the target of invalid symbolic references makes the output concise (compared to printing individual instances).

```
(Target Class X) having problem XXX. referenced by
(Source Class A)
(Source Class B)
```

However this format does not work with `AbstractMethodProblem`. As of now, we separate `AbstractMethodProblem` from other problems (using `instanceof`) and print them individually.

Can polymorphism help the grouping LinkageProblems in a better way?