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

[Mark de Wever via cfe-commits]

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

Thanks for working in this.

I didn't start the CI jobs since I know they will fail.

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