[llvm] [libcxx] Add libc++ github actions workflow to replace buildkite (PR #71836)

[via llvm-commits]

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EricWF wrote:

Yes, I just need to figure out how to use it.

We neet to use this for phabricator triggered CI runs, and the other one when triggered from github. 

https://github.com/llvm/llvm-project/pull/71836