The goal of this refactoring effort was to better understand how interestingness was propagated in BugReporter.cpp, which eventually turned out to be a dead end, but with such a twist, I wouldn't even want to spoil it ahead of time. However, I did get to learn a lot about how things are working in there.
In these series of patches, as well as cleaning up the code big time, I invite you to study how BugReporter.cpp operates, and discuss how we could design this file to reduce the horrible mess that it is.
This patch reverts a great part of rC162028, which holds the title "Allow multiple PathDiagnosticConsumers to be used with a BugReporter at the same time.". This, however doesn't imply that there's any need for multiple "layers" or stacks of interesting symbols and regions, quite the contrary, I would argue that we would like to generate the same amount of information for all output types, and only process them differently.