[libcxx] [llvm] [libc++] Move some macOS CI jobs to Github actions (PR #89083)

[Louis Dionne via llvm-commits]

ldionne wrote:

> How often will this job be running? We are limited to only 5 concurrent macOS jobs at a time, so if this is going to be running for each PR, we may quickly run out of capacity.

This would run after stage1 has completed. Only a subset of all PRs make it there, as most will fail in stage1.

> If everything is already running on machines owned by the foundation, it should just be a matter of setting up the Github actions agent on them to get more runner availability? Like @tstellar mentioned, that's not what this PR does currently though.

Yes, we could do that or we could pay for additional capacity on Github directly. I have been maintaining the Foundation macOS nodes and not having to do that anymore is the main motivation for this change.

https://github.com/llvm/llvm-project/pull/89083