Add post-commit job to cleanup backport branches (#3176)

Signed-off-by: Sidorov, Dmitry <[email protected]>

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@c846c0516e3ff64

Author: MrSidims

llvm-spirv/.github/workflows/cleanup-backport-branches.yml

@@ -0,0 +1,38 @@
+name: Cleanup Backport Branches
+
+on:
+  pull_request:
+    types: [closed]
+
+permissions:
+  contents: write
+
+jobs:
+  cleanup:
+    if: >
+      github.event.pull_request.head.repo.full_name == github.repository &&
+      startsWith(github.event.pull_request.head.ref, 'backport/pr-')
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Delete branch
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            const branch = context.payload.pull_request.head.ref;
+            try {
+              await github.rest.git.deleteRef({
+                owner: context.repo.owner,
+                repo:  context.repo.repo,
+                ref:   `heads/${branch}`
+              });
+              console.log(`Success. Deleted '${branch}'`);
+            } catch (error) {
+              if (error.status === 422) {
+                console.log(`Branch '${branch}' not found`);
+              }
+            } else {
+              throw error;
+            }
+          }