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

[via llvm-commits]

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@@ -0,0 +1,112 @@
+name: Build and Test libc++
+
+on: pull_request
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+
+env:
+  CMAKE: "/opt/bin/cmake"
+
+# Comment
+jobs:
+  stage1:
+    runs-on: libcxx-runners-16
+    continue-on-error: false
+    strategy:
+      fail-fast: true
+      matrix:
+        config: [ 'generic-cxx26', 'generic-cxx03', 'generic-modules' ]
+        cc: [ 'clang-18' ]
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EricWF wrote:

Done. Not sure how the expansion works. But I'll figure it out.

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