[llvm] [CI] Upload JUnit Test Results as Artifacts (PR #141905)

[David Spickett via llvm-commits]

https://github.com/DavidSpickett approved this pull request.

I could swear they were artifacts already, or were at one point. Anyway, sounds like a good idea to me.

> This is intended to enable developing a tool that can spot flaky tests.

Looking forward to this!

https://github.com/llvm/llvm-project/pull/141905