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

[via llvm-commits]

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@@ -0,0 +1,137 @@
+name: Build and Test libc++
+
+on: pull_request
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+permissions:
+  checks: write
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EricWF wrote:

All of the permissions have been removed and it seems to work great.

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