[llvm] [ADT] Add unit tests for set_subtract (PR #99561)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `arc-builder` running on `arc-worker` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/3/builds/1731

Here is the relevant piece of the build log for the reference:
```
Step 5 (build-unified-tree) failure: build (failure)

```

https://github.com/llvm/llvm-project/pull/99561