`ProblemReport.clone()` should perform a shallow copy

[stanislav-tkach]

Summary

Fix the ProblemReport Clone implementation, so it behaves as wrapped in Arc<Mutex> (has shared interior mutability).

Description

Currently ProblemReport::clone performs a deep copy because ConcurrentVec that is used inside behaves this way. We don't want such behavior for ProblemReport and it is unlikely that ConcurrentVec will be changed, so we need to do that on our side.