Greatly simplify the logic in ExplodedGraphImpl::TrimGraph. Now we just do a

vanilla reverse-BFS followed by a forward-DFS instead of resulting to strange
histrionics (whose purpose I can no longer remember) in the reverse-BFS stage.
This fixes an assertion failure in BugReporter due to edge cases where no root
was being hit in the reverse-BFS phase.

llvm-svn: 65160