[llvm] [ci] Handle the case where all reported tests pass but the build is still a failure (PR #120264)

[David Spickett via llvm-commits]

DavidSpickett wrote:

This is all done within the "if we have a buildkite agent", so it should not disrupt work on the new system.

It's getting a bit complex, but (famous last words) I think this is the last combination it needs to handle, as shown in the table.

https://github.com/llvm/llvm-project/pull/120264