[analyzer] Eliminate InterExplodedGraphMap class and NodeBackMap typedef.

...in favor of this typedef:

  typedef llvm::DenseMap<const ExplodedNode *, const ExplodedNode *>
          InterExplodedGraphMap;

Use this everywhere the previous class and typedef were used.

Took the opportunity to ArrayRef-ize ExplodedGraph::trim while I'm at it.

No functionality change.

llvm-svn: 177215