[analyzer] On eliminating BugReporterVisitors

Just wanted to attract a bit more attention to my proposal in ⚙ D58367 [analyzer] NFC: Improve upon the concept of BugReporterVisitor. - it's an improved checker API that eliminates the need to write bug reporter visitors in checkers, but instead enables squeezing note-generating code directly into addTransition.

This is cool because the message is generated within the part of code that already has all the information that's necessary to generate the message, which is much easier than reverse-engineering what happened "a posteriori" in order to re-construct that information. This doesn't really remove the old API and i don't see any obvious downsides.