[analyzer] C++ objects returned on the stack may be wrapped in ExprWithCleanups.

In C++, objects being returned on the stack are actually copy-constructed into
the return value. That means that when a temporary is returned, it still has
to be destroyed, i.e. the returned expression will be wrapped in an
ExprWithCleanups node. Our "returning stack memory" checker needs to look
through this node to see if we really are returning an object by value.

PR13722

llvm-svn: 162817