Simplify ownership of ExplodedGraph in the CoreEngine by removing unique_ptr indirection.

Summary: I was going to fix the use of raw pointer ownership in "takeGraph" when I realized that function was unused and the whole ExplodedGraph could just be owned by value without the std::unique_ptr indirection at all.

Reviewers: jordan_rose

Differential Revision: http://reviews.llvm.org/D4833