[clang] [clang] add test verifying DR242 behavior (PR #201441)

[Jess Ding via cfe-commits]

jessding wrote:

> Please reply to this message to confirm that you have read these policies, especially the LLVM AI Tool Use Policy, and that any AI tool usage has been noted in the PR description.

I confirm that I've read these policies! AI usage is noted in my PR description above.

https://github.com/llvm/llvm-project/pull/201441