workflows/premerge: Cancel in progress jobs when a PR is merged (#125329) (#125588)

(cherry picked from commit 2deba08)

Author: tstellar

.github/workflows/premerge.yaml

@@ -5,14 +5,27 @@ permissions:
 
 on:
   pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      # When a PR is closed, we still start this workflow, but then skip
+      # all the jobs, which makes it effectively a no-op.  The reason to
+      # do this is that it allows us to take advantage of concurrency groups
+      # to cancel in progress CI jobs whenever the PR is closed.
+      - closed
+    paths:
+      - .github/workflows/premerge.yaml
   push:
     branches:
       - 'main'
       - 'release/**'
 
 jobs:
   premerge-checks-linux:
-    if: false && github.repository_owner == 'llvm'
+    if: >-
+        false && github.repository_owner == 'llvm' &&
+        (github.event_name != 'pull_request' || github.event.action != 'closed')
     runs-on: llvm-premerge-linux-runners
     concurrency:
       group: ${{ github.workflow }}-linux-${{ github.event.pull_request.number || github.sha }}
@@ -71,7 +84,9 @@ jobs:
           ./.ci/monolithic-linux.sh "$(echo ${linux_projects} | tr ' ' ';')" "$(echo ${linux_check_targets})" "$(echo ${linux_runtimes} | tr ' ' ';')" "$(echo ${linux_runtime_check_targets})"
 
   premerge-checks-windows:
-    if: false && github.repository_owner == 'llvm'
+    if: >-
+        false && github.repository_owner == 'llvm' &&
+        (github.event_name != 'pull_request' || github.event.action != 'closed')
     runs-on: llvm-premerge-windows-runners
     concurrency:
       group: ${{ github.workflow }}-windows-${{ github.event.pull_request.number || github.sha }}
@@ -139,7 +154,8 @@ jobs:
     if: >-
       github.repository_owner == 'llvm' &&
       (startswith(github.ref_name, 'release/') ||
-       startswith(github.base_ref, 'release/'))
+       startswith(github.base_ref, 'release/')) &&
+      (github.event_name != 'pull_request' || github.event.action != 'closed')
     steps:
       - name: Checkout LLVM
         uses: actions/checkout@v4