[llvm] [workflows] Build a container for running CI on github actions (PR #75286)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

> Rather than having one container that can be used for every job, I created a container that only works with the llvm-project-tests workflows (and probably the release-binaries workflow too). I'm still not convinced that having different containers for each job is the right approach, but these workflow are the only jobs that actually need a container right now, because I want to move them to the self-hosted runner. The rest of the jobs are fine on the GitHub runners, so they don't really need to use containers.

That works. My point was mainly for jobs with extensive python dependencies, but we can look at that later, and that's also only if we want to move those jobs to containers given that they are cheap enough to run on the github hosted runners.

> I installed the most recent clang release (with binaries from the release page) into the container and added its install directory to PATH. I'm planning on enabling PGO/llvm bolt optimizations for the release builds in the next cycle, so using the release binaries will eventually give us a highly optimized clang for the container as you suggested.

Sounds good.

> I added unique tags for each new container build.

Awesome!

Everything in this patch pretty much LGTM. Just minor nits and I don't want to keep anything blocked for future work.

https://github.com/llvm/llvm-project/pull/75286