[libc++] Investigate performance regression in "[libc++] Refactor memory allocation in basic_string"

We've introduced a performance regression in https://github.com/llvm/llvm-project/pull/128423. https://github.com/llvm/llvm-project/pull/164718 fixed at least some of it. We should check whether there are any other functions where we regressed.

Start with PR #128423 and the partial fix in PR #164718, then inspect the libc++ basic_string memory-allocation changes and compare the affected functions for remaining regressions. Done means determining whether other functions also regressed and recording the investigation results.

Written by the indexing model from the issue text.