[workflows] Add a new workflow for testing release branch CI

Since we commit all changes to the release branch CI to main first, we
need someway to test that these changes to main don't break the CI.

Author: tstellar

.github/workflows/llvm-project-workflow-tests.yml

@@ -0,0 +1,31 @@
+# This workflow will test the llvm-project-tests workflow in PRs
+# targetting the main branch.  Since this workflow doesn't normally
+# run on main PRs, we need some way to test it to ensure new updates
+# don't break it.
+
+name: LLVM Workflow Test
+
+permissions:
+  contents: read
+
+on:
+    branches:
+      - 'main'
+    paths:
+      - '.github/workflows/llvm-project-tests.yml'
+      - '.github/workflows/llvm-project-workflow-tests.yml'
+
+concurrency:
+  # Skip intermediate builds: always.
+  # Cancel intermediate builds: only if it is a pull request build.
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+
+jobs:
+  llvm-test:
+    if: github.repository_owner == 'llvm'
+    name: Build and Test
+    uses: ./.github/workflows/llvm-project-tests.yml
+    with:
+      build_target: check-all
+      projects: clang;lld;libclc;lldb