Deal with libc++ `std::expected` ABI breakage

[giordano]

LLVM accidentally broke the ABI of std::expected in libc++ v16: https://discourse.llvm.org/t/abi-break-in-libc-for-a-17-x-guidance-requested/74483. We may have to deal with this split at some point, depending on what ABI systems like macOS and FreeBSD will use in the future.