fix: Create separate report_clang_format workflow in master branch

The initial version of this workflow just uses `console.log` to report
the context of the `context` object.  It is hoped that there will
be enough information in this context to identify the PR to comment on,
without the `check_clang_format` workflow having to upload a
'build artefact' of some kind - see example of what I hoe to avoid
starting at `ReceivePR.yml` here:

https://securitylab.github.com/research/github-actions-preventing-pwn-requests/

A follow-up PR will (if possible) add the code to create comments
when `check_clang_format` fails.

Part of google#5659.

.github/workflows/report_clang_format.yml

@@ -0,0 +1,31 @@
+name: Report clang format
+
+# Runs after the Check clang format workflow (check_clang_format.yml) to
+# post a helpful comment to the PR if the check failed.
+#
+# N.B.: Per https://github.saobby.my.eu.orgmunity/t/workflow-runs-not-starting-after-previous-workflow-completes/128345/8
+# only the version of this workflow on the master (default) branch
+# will be run, regardless of what branch is targetted by the PR being
+# checked by check_clang_format.yml.
+#
+# N.B.: Runs with a read-write repo token.  Do not check out the
+# submitted branch!
+on:
+  workflow_run:
+    workflows: ["Check clang format"]
+    types:
+      - completed
+
+on: [pull_request]
+
+jobs:
+  on-failure:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.workflow_run.conclusion == 'failure' }}
+    steps:
+    - uses: actions/github-script@v5
+      with:
+        script: |
+          // Not sure yet how to find out which PR triggered the run, so
+          // dump the whole context object to try to find useful information.
+          console.log('%o', context);