[analyzer] Add debug helper LocationContext::dumpStack().

Sample output:
  #0 void construct(pointer __p, llvm::ImutAVLTree<llvm::ImutContainerInfo<clang::ento::BugType *> > *const &__val)
  #1 void push_back(const value_type &__x)
  #2 void destroy()
  #3 void release()
  #4 void ~ImmutableSet()
[analyzer] Handle caching out while evaluating a C++ new expression.

Evaluating a C++ new expression now includes generating an intermediate
ExplodedNode, and this node could very well represent a previously-
reachable state in the ExplodedGraph. If so, we can short-circuit the
rest of the evaluation.

Caught by the assertion a few lines later.

<rdar://problem/13510065>
[analyzer] Restructure ExprEngine::VisitCXXNewExpr to do a bit less work.

No functionality change.