[libc++] Trigger the BuildKite pipeline from Github Actions

Instead of always triggering the BuildKite pipeline and then
determining whether there is anything to do, trigger it from
the libc++ CI Github action. Since that Github action is only
triggered when relevant files have been modified, this simplifies
the triggering logic.

Importantly, it will also prevent all of the monorepo CI jobs from
waiting on the Buildkite pipeline to report that there's nothing
to do.

Author: ldionne

.github/workflows/libcxx-build-and-test.yaml

@@ -288,3 +288,16 @@ jobs:
       - name: Build and test
         run: |
           bash libcxx/utils/ci/run-buildbot ${{ matrix.config }}
+
+  buildkite_platforms:
+    # needs: [ stage2 ] # TODO: temporarily always trigger
+    runs-on: llvm-premerge-libcxx-next-runners
+    steps:
+      - name: Trigger Buildkite pipeline
+        uses: buildkite/trigger-pipeline-action@v2.4.0
+        with:
+          buildkite_api_access_token: ${{ secrets.BUILDKITE_API_ACCESS_TOKEN }}
+          pipeline: llvm-project/libcxx-ci
+          branch: ${{ github.ref_name }}
+          commit: ${{ github.sha }}
+          message: "Triggered by GitHub Actions PR against ${{ github.ref_name }}"