[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].
+#
+# 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.
+#
+# Stages 1 & 2 are meant to be "smoke tests", and are meant to catch most build/test failures quickly and without using
+# too many resources.
+# Stage 3 is "everything else", and is meant to catch breakages on more niche or unique configurations.
+#
+# Therefore, we "fail-fast" for any failures during stages 1 & 2, meaning any job failing cancels all other running jobs,
+# under the assumption that if the "smoke tests" fail, then the other configurations will likely fail in the same way.
+# However, stage 3 does not fail fast, as it's more likely that any one job failing is a flake or a configuration-specific
+#
+name: Build and Test libc++
+on:
+  pull_request:
+    paths:
+      - 'libcxx/**'
+      - 'libcxxabi/**'
+      - 'libunwind/**'
+      - 'runtimes/**'
+      - 'cmake/**'
+      - '.github/workflows/libcxx-build-and-test.yaml'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+
+env:
+  CMAKE: "/opt/bin/cmake"
+  # LLVM POST-BRANCH bump version
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ldionne wrote:

Add a TODO since we're not using those yet?

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