Stack hints are attached to PathDiagnosticEventPieces in order to improve path notes at the call site for the call in which the event has occured. For example, they are currently used only by MallocChecker in order to produce the fancy "Returning allocated memory"/"Returning; memory was released" note. By the way, in fact nobody else currently uses this functionality - only MallocChecker.
I want to make the PathDiagnostic interface completely ignorant of Static Analyzer specific concepts such as ExplodedNode or SymbolRef, so i moved this interface to BugReporter. Stack hints are now owned by the PathSensitiveBugReport object against which the visitor emits event pieces for which it needs stack hints.
I'm open to discuss a better design here. Eg., i thought about making it part of the visitor interface instead, but i don't immediately see how to do this without breaking the logic of "only add the note at the call site in which the event has happened, not every time allocated memory is returned from anywhere".