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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Notes to reviewers:
* This is based on https://github.com/llvm/llvm-project/pull/113160.
* The first commit is that previous PR.
* The second commit is the new script.
* The third commit is some example changes so I can show you what the reports look like, they will be removed if this is approved.

This is an alternative to forking the existing [Buildkite plugin](https://github.com/buildkite-plugins/junit-annotate-buildkite-plugin). I do not have examples of those results because it's can't run without docker, but I do have WIP changes to the scripting to get that far (https://github.com/llvm/llvm-project/pull/113290) if you want co compare.

There are a couple of other things we could do:
* Print the report to stdout as well so it is part of the logfile for download. However it will have a bunch of markdown stuff in it.
* Or, write the report to `test_results.md` and make it an artifact of the job. The annotation can then say "you can download this report from the artifacts...".

I think the current state of this PR is a big improvement as it is, and I am able to commit more time to improving it as time goes on and if this needs to be ported to GitHub infrastructure later.

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