[llvm] Clarify conditions of `lit` exiting with exit code 1 (PR #136190)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

You might want to tweak the PR description given this revised version of the patch. Otherwise, LGTM. Do you need me to merge this for you?

https://github.com/llvm/llvm-project/pull/136190