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

[Louis Dionne via llvm-commits]

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@@ -0,0 +1,112 @@
+name: Build and Test libc++
----------------
ldionne wrote:

I would like us to add a comment that explains a few aspects of this setup, because some of these choices are not arbitrary. Something like this:

```
# This file defines pre-commit CI for libc++ [bla bla bla].
#
# We split the configurations in multiple stages with the intent of saving compute time
# when a job fails early in the pipeline. This is why the jobs are marked as `continue-on-error: false`.
# We try to run the CI configurations with the most signal in the first stage.
# 
# TODO: Explain the fail-fast behavior (enabled for the first stage but not the other ones).
```

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