Add a generic Process method to dump plugin history.

I need to call this to figure out why the assert in
StopInfoMachException::CreateStopReasonWithMachException is triggering, but
it isn't appropriate to directly access the GDBRemoteCommunication there.  And
dumping whatever history the process plugin has collected during the run isn't
gdb-remote specific...

Differential Revision: https://reviews.llvm.org/D154992