warn unreachable on primer

Author: hauntsaninja

.github/workflows/mypy_primer.yml

@@ -65,7 +65,7 @@ jobs:
             --new $GITHUB_SHA --old base_commit \
             --num-shards 6 --shard-index ${{ matrix.shard-index }} \
             --debug \
-            --additional-flags="--debug-serialize" \
+            --additional-flags="--debug-serialize --warn-unreachable" \
             --output concise \
             --mypy-install-librt \
             | tee diff_${{ matrix.shard-index }}.txt