[analyzer] Exploration strategy prioritizing unexplored nodes first

See D42775 for discussion.  Turns out, just exploring nodes which
weren't explored first is not quite enough, as e.g. the first quick
traversal resulting in a report can mark everything as "visited", and
then subsequent traversals of the same region will get all the pitfalls
of DFS.
Priority queue-based approach in comparison shows much greater
increase in coverage and even performance, without sacrificing memory.

Differential Revision: https://reviews.llvm.org/D43354

llvm-svn: 326136