[analyzer] Eliminate InterExplodedGraphMap class and NodeBackMap typedef.

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177215 91177308-0d34-0410-b5e6-96231b3b80d8