Recommit rL245802: Cleanup fancy pointer rebinding in list using __rebind_pointer.

Currently we need an #ifdef branch every time we use pointer traits to rebind a pointer because
it is done differently in C++11 and C++03. This patch introduces the __rebind_pointer utility to
clean this up.

Also add a test that list and it's iterators can be instantiated with incomplete element types.

llvm-svn: 245806