Add 'StoreRef' smart pointer to allow more fine-grain memory lifetime control of Store objects.

This yields a minor memory reduction (for larger functions) on Sqlite at the cost of slightly
higher memory usage on some functions because of the increased size of GRState (which can be optimized).

I expect the real memory savings from this enhancement will come when we aggressively
canabilize more of the ExplodedGraph.

llvm-svn: 126012