Implement GitHub Actions for lint and buildAndTest

[GleasonK]

Initial implementation of GitHub Actions CI. This changelist implements two actions:

Build & Test:

• Check out the llvm revision specified in llvm_version.txt
• Build LLVM with ccache enabled (first time cost ~1.5hr, following runs ~10-15min)
• Build StableHLO with ccache
• Run ninja check-stablehlo
• Fails if any build or test failures.

Lint:

• Run git-clang-format on the files being submitted.
• Fails if any files' formatting are updated.

Examples of what the CI page will look like can be seen at: https://github.com/GleasonK/stablehlo/actions

Future work:

• We can consider running buildAndTest on other platforms in the future, github offers macos/mac-arm.
• The lint action worked in the limited testing that I ran on it, but could use a bit more testing. I think that the production repo will be a good way to test if everything is in order.
• Consider any other tests/linting we want to run. If we add python bindings, there is some python formatting and linting that can be added later.

Closes #12

[GleasonK]

Addressed feedback. Actions job with the new changes is running on https://github.com/GleasonK/stablehlo/actions. Let's wait for that before any accept/merge is made.

Edit: Tests all passed in gleasonk/stablehlo actions.

[GleasonK]

Rebased on main.

Should be good for final review / merge. Please wait for the actions job on my forked repo to complete successfully before merge, the lint action already passed:
https://github.com/GleasonK/stablehlo/actions/runs/2972714760

[GleasonK]

Edit: GH Actions job passed. Only took 4m 53s with caching, not bad.

[burmako]

Please also update the PR description to reword "We can consider only running buildAndTest on linux in the future, mac is a bit of a redundancy, but will help us distinguish a real failure from a build configuration issue in GH Actions".