[llvm] Upstream libc++ buildbot restarter. (PR #93582)

[Tom Stellard via llvm-commits]

================
@@ -0,0 +1,108 @@
+name: Restart Preempted Libc++ Workflow
+
+# The libc++ builders run on preemptable VMs, which can be shutdown at any time.
+# This workflow identifies when a workflow run was canceled due to the VM being preempted,
+# and restarts the workflow run.
+
+# We identify a canceled workflow run by checking the annotations of the check runs in the check suite,
+# which should contain the message "The runner has received a shutdown signal."
+
+# Note: If a job is both preempted and also contains a non-preemption failure, we do not restart the workflow.
+
+on:
+  workflow_run:
+    workflows:
+      - "Build and Test libc\+\+"
+    types:
+      - failure
+      - canceled
+
+permissions:
+  contents: read
+
+jobs:
+  restart:
+    name: "Restart Job"
----------------
tstellar wrote:

We need to disable this workflow for forks, so
` if: github.repository_owner == 'llvm'`

https://github.com/llvm/llvm-project/pull/93582