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

[via llvm-commits]

================
@@ -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
----------------
EricWF wrote:

I think it's vestigial. Removing.

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