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

[Louis Dionne via llvm-commits]

ldionne wrote:

> @ldionne This PR seems to have a lot of unrelated libc++ changes that somehow got pulled in?

I am just using this to iterate on fixing flaky tests that only show up on the Github-provided macOS builders. These changes won't be part of the final patch.

> 
> Also, we'll have to think about the capacity for this. I believe we can only have five concurrent macOS jobs running at the same time. I'm not sure how many concurrent jobs the libc++ CI normally runs or how long the macOS jobs run for, but this could quickly end up becoming an issue. I don't think it would block too much else in the project though as everything (that I can think of) runs on Linux runners, other than the release jobs.

I agree. We'd probably want to set up the existing Foundation-provided macOS machines as runners available via Github actions, but I wanted to look into that separately.

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