[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()

llvm-svn: 178400