[Infrastructure] Set up a dependent PR check (#343)

https://github.com/marketplace/actions/pr-dependency-check

Fixes #337

.github/workflows/dependent-pr.yaml

@@ -0,0 +1,16 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: Dependencies
+
+on: [pull_request]
+
+jobs:
+  check_dependencies:
+    runs-on: ubuntu-latest
+    name: check PRs landed
+    steps:
+      - uses: gregsdennis/dependencies-action@main
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}