Renamed SimulVertex, SimulGraph, and SimulEngine to:

  ExplodedNode, ExplodedGraph  (to match the vocabulary in the RHS paper)
  ReachabilityEngine
  
The implementation of the core of the path-sensitive dataflow solver has
been de-templatized and places in ReachabilityEngine.cpp.

The solver is still incomplete.

llvm-svn: 45711