For logical backtrace work, lldb needs to track Module unloads etc & symoblicate an address based on a point in time

<rdar://problem/15314403>

This patch adds a new lldb_private::SectionLoadHistory class that tracks what shared libraries were loaded given a process stop ID. This allows us to keep a history of the sections that were loaded for a time T. Many items in history objects will rely upon the process stop ID in the future.