[llvm] [libcxx] [clang-tools-extra] [clang] [libc++][span] P2821R5: span.at() (PR #74994)

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 requested changes to this pull request.

Looks mostly good, but I have a few small comments.

https://github.com/llvm/llvm-project/pull/74994