Optimize std::__tree::__assign_multi to insert the provided range at the end of the tree every time (#131030)

This improves performance for the copy-assignment operators of associative containers such as `std::map`.

This optimization already exists in other places in the codebase, and seems to have been missed here.

GitOrigin-RevId: 5f9164978b218fa65c53a59e91ea01831735f7a4
1 file changed