[CI][Github] Add linux premerge workflow

.github/workflows/premerge.yaml

@@ -0,0 +1,66 @@
+name: LLVM Premerge Checks
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    paths:
+      - .github/workflows/premerge.yaml
+
+jobs:
+  premerge-checks-linux:
+    if: github.repository_owner == 'llvm'
+    runs-on: llvm-premerge-linux-runners
+    concurrency:
+      group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+      cancel-in-progress: true
+    container:
+      image: ghcr.io/llvm/ci-ubuntu-22.04:latest
+    defaults:
+      run:
+        shell: bash
+    steps:
+      - name: Checkout LLVM
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
+      - name: Setup ccache
+        uses: hendrikmuhs/[email protected]
+      - name: Build and Test
+        run: |
+          git config --global --add safe.directory '*'
+
+          modified_files=$(git diff --name-only HEAD~1...HEAD)
+          modified_dirs=$(echo "$modified_files" | cut -d'/' -f1 | sort -u)
+
+          echo $modified_files
+          echo $modified_dirs
+
+          . ./.ci/compute-projects.sh
+
+          all_projects="bolt clang clang-tools-extra compiler-rt cross-project-tests flang libc libclc lld lldb llvm mlir openmp polly pstl"
+          modified_projects="$(keep-modified-projects ${all_projects})"
+
+          linux_projects_to_test=$(exclude-linux $(compute-projects-to-test 0 ${modified_projects}))
+          linux_check_targets=$(check-targets ${linux_projects_to_test} | sort | uniq)
+          linux_projects=$(add-dependencies ${linux_projects_to_test} | sort | uniq)
+
+          linux_runtimes_to_test=$(compute-runtimes-to-test ${linux_projects_to_test})
+          linux_runtime_check_targets=$(check-targets ${linux_runtimes_to_test} | sort | uniq)
+          linux_runtimes=$(echo ${linux_runtimes_to_test} | sort | uniq)
+
+          if [[ "${linux_projects}" == "" ]]; then
+            echo "No projects to build"
+            exit 0
+          fi
+
+          echo "Building projects: ${linux_projects}"
+          echo "Running project checks targets: ${linux_check_targets}"
+          echo "Building runtimes: ${linux_runtimes}"
+          echo "Running runtimes checks targets: ${linux_runtime_check_targets}"
+
+          export CC=/opt/llvm/bin/clang
+          export CXX=/opt/llvm/bin/clang++
+
+          ./.ci/monolithic-linux.sh "$(echo ${linux_projects} | tr ' ' ';')" "$(echo ${linux_check_targets})" "$(echo ${linux_runtimes} | tr ' ' ';')" "$(echo ${linux_runtime_check_targets})"