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

[Thurston Dang via cfe-commits]

thurstond 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.

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