[llvm] [ci] New script to generate test reports as Buildkite Annotations (PR #113447)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I've split up the code:
Hide the lit option - https://github.com/llvm/llvm-project/pull/114812
Write results to unique file names - https://github.com/llvm/llvm-project/pull/113160 (also the first commit in this PR, so that the builds pass)
This PR, which adds the reporting script.

https://github.com/llvm/llvm-project/pull/113447