Use the correct data structures!

ExplodedGraph::TrimGraph:
- Just do a DFS both ways instead of BFS-DFS. We're just determining what subset
  of the nodes are reachable from the root and reverse-reachable from the bug
  nodes.  DFS is more efficient for this task.
  
BugReporter:
- MakeReportGraph: Do a reverse-BFS instead of a reverse-DFS to determine the
  approximate shortest path through the simulation graph. We were seeing some
  weird cases where too many loops were being reported for simple bugs. Possibly
  we will need to replace this with actually computing the shortest path in
  terms of line numbers.

llvm-svn: 66842