[analyzer] Break cycles (optionally) when trimming an ExplodedGraph.

Having a trimmed graph with no cycles (a DAG) is much more convenient for
trying to find shortest paths, which is exactly what BugReporter needs to do.

Part of the performance work for <rdar://problem/13433687>.
[analyzer] Re-apply "Do part of the work to find shortest bug paths up front".

With the assurance that the trimmed graph does not contain cycles,
this patch is safe (with a few tweaks), and provides the performance
boost it was intended to.

Part of performance work for <rdar://problem/13433687>.