ci: add github action for running coverage

.github/workflows/llvm_cov.yml

@@ -0,0 +1,15 @@
+---
+name: Run llvm-cov
+on:  # yamllint disable-line rule:truthy
+  pull_request:
+    branches: ['*']
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v3
+
+      - name: Run Coverage Script
+        run: bazel run //tools/coverage:llvm_cov -- -t //examples/cpp:test