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

[Tom Stellard via llvm-commits]

tstellar wrote:

@boomanaiden154 A few updates based on your suggestions:

- 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.
- 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.
- I added unique tags for each new container build.

TODOs:
- Choose a better container name?
- Add workflow_dispatch. 

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