[analyzer] Fix ExprEngine::ViewGraph to handle C++ initializers.

Debugging aid only, no functionality change.
[analyzer] Fix test to actually test what was intended.
[analyzer] Use a forward BFS instead of a reverse BFS to find shortest paths.

For a given bug equivalence class, we'd like to emit the report with the
shortest path. So far to do this we've been trimming the ExplodedGraph to
only contain relevant nodes, then doing a reverse BFS (starting at all the
error nodes) to find the shortest paths from the root. However, this is
fairly expensive when we are suppressing many bug reports in the same
equivalence class.

r177468-9 tried to solve this problem by breaking cycles during graph
trimming, then updating the BFS priorities after each suppressed report
instead of recomputing the whole thing. However, breaking cycles is not
a cheap operation because an analysis graph minus cycles is still a DAG,
not a tree.

This fix changes the algorithm to do a single forward BFS (starting from the
root) and to use that to choose the report with the shortest path by looking
at the error nodes with the lowest BFS priorities. This was Anna's idea, and
has the added advantage of requiring no update step: we can just pick the
error node with the next lowest priority to produce the next bug report.

<rdar://problem/13474689>
Revert "[analyzer] Break cycles (optionally) when trimming an ExplodedGraph."

The algorithm used here was ridiculously slow when a potential back-edge
pointed to a node that already had a lot of successors. The previous commit
makes this feature unnecessary anyway.

This reverts r177468 / f4cf6b10f863b9bc716a09b2b2a8c497dcc6aa9b.

Conflicts:

	lib/StaticAnalyzer/Core/BugReporter.cpp