[clang] Fix test added in 849de61 to work when the compiler defaults to C++20. (PR #193361)

[Jan Korous via cfe-commits]

https://github.com/jkorous-apple approved this pull request.

Thank you for improving the tests!

The note is intentionally not emitted for pre-C++20 language modes as the suggestions rely on `std::span` which was introduced by C++20.

LGTM

https://github.com/llvm/llvm-project/pull/193361