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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Minor comments.

I'm assuming the plan is to migrate the key existing workflows to use the container image after this lands? We might also want to add a couple more dependencies or sub-containers (that use this one as their base image) for specific workflows as some of them (like the code formatting action) install packages through `apt-get` (or could).

Installing python packages into a container would be nice as well for some of the jobs and would shave off a bit of setup time, but that's already mostly negated by the caching.

For building LLVM/running tests, we'll probably need to make some changes to take more explicit control of the toolchain that we're using. For a build-once run many times container, I think a ThinLTO+PGO+BOLTed clang based toolchain would make a lot of sense, but that is irrelevant for this patch.


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