[llvm] [libc++] Add assertions for potential OOB reads in std::nth_element (PR #67023)

[Daniel Kutenin via llvm-commits]

danlark1 wrote:

>Also @danlark1 if you rebase onto main the issues with macOS in the CI should be fixed now.

@ldionne, Done

https://github.com/llvm/llvm-project/pull/67023