[clang] [DO not merge] demo test for https://github.com/llvm/llvm-project/pull/140529 (PR #140602)

[Thurston Dang via cfe-commits]

thurstond wrote:

This patch should be "[NFC] Precommit test for src:*=sanitize for UBSan" (or similar), have the test output "passing" for the current build of clang (i.e., probably "incorrect" output), reviewed and then merged.

https://github.com/llvm/llvm-project/pull/140602