[libc++] Fix use-after-free when building with _LIBCPP_DEBUG=1
The issue is the following code:
__cn1->__add(*__ip); (*__ip)->__c_ = __cn1;
__ip points into the array of iterators for container __cn2. This code adds
the iterator to the array of iterators for __cn1, and updates the iterator to
point to the new container.
This code works fine, except when __cn1 and __cn2 are the same container.
__cn1->__add() might need to grow the array of iterators, and when it does,
__ip becomes invalid, so the second line becomes a use-after-free error.
Simply swapping the order of the above two lines is not sufficient, because of
the memmove() below. The easiest and most performant solution is just to skip
touching any iterators if the containers are the same.
Differential Revision: https://reviews.llvm.org/D58926