[libc++] Investigate why `std::vector<std::string>::operator=` is 50% slower with the stable ABI

I suspect that is related to the set of external instantiations in `std::string`.

Start by investigating the reported performance difference in libc++ for std::vector::operator= with the stable ABI. Compare the relevant behavior with and without the stable ABI, then examine whether external instantiations in std::string account for the slowdown. Done means identifying the cause and documenting a concrete direction for addressing it.

Written by the indexing model from the issue text.