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

[Louis Dionne via llvm-commits]

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+# This file defines pre-commit CI for libc++ [bla bla bla].
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ldionne wrote:

I intended you to fill something in instead of `[bla bla bla]` if there's something else to say 😅. If there's nothing else to say, we can just drop that too.

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