[libcxx] [llvm] 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++
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+on: pull_request
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ldionne wrote:

We need a path selector to only trigger on the following paths: `libcxx|libcxxabi|libunwind|runtimes|cmake`

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