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

[Aiden Grossman 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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boomanaiden154 wrote:

Can we set the permissions to read only at the top level of the file and add additional permissions at the job level? That's what we're trying to do across the rest of the project to help reduce the possibility of mistakenly adding permissions where they aren't needed.

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