[clang] [clang-tools-extra] [llvm] [libcxx] [libc] [lldb] [flang] [openmp] [mlir] [compiler-rt] [libc++][span] P2821R5: span.at() (PR #74994)

[Mark de Wever via cfe-commits]

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

Mostly looks good, a few comments.

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