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

[Vitaly Buka via cfe-commits]

vitalybuka wrote:

> > The linked pull request (#140529) introduces a new feature rather than correcting a bug.
> 
> I would argue that since the `src:*=sanitize` syntax could reasonably be expected to work, but it did not work, then that is both a missing feature and a bug (not working as intended).
> 
> If it had been a new syntax that no user would expect to work (`newfoo:*`, or `=newblah`) then that would be a missing feature but not a bug.

Yep, feature or bug, but the current behavior is not desirable.
It's nice when test shows a difference in behavior, not just the final result.

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