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

[Louis Dionne via llvm-commits]

================
@@ -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:
----------------
ldionne wrote:

We seem to have removed some of the env vars that we used to have:

```
_common_env: &common_env
      ENABLE_CLANG_TIDY: "On"
      LLVM_SYMBOLIZER_PATH: "/usr/bin/llvm-symbolizer-${LLVM_HEAD_VERSION}"
      CLANG_CRASH_DIAGNOSTICS_DIR: "crash_diagnostics"
```

Those are important, in particular the clang tidy enablement.

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