Clinical trial flops have become the norm for Alzheimer’s disease treatments, despite the billions put into research for the memory-robbing disorder. But a new Lexington, MA-based startup called Alzheon sees opportunity in all of those high-profile failures of the past. Its belief is that these failures have left behind a treasure trove of clinical data, and stalled drug candidates, that can be plucked, refined, and developed to work for more specific groups of patients.