[rust] Add CI Checks to Rust fork

- On Linux, run `check-llvm` and `check-lld`.
- On macOS, run `check-llvm`.

Author: dianqk

.github/workflows/rust-premerge.yaml

@@ -0,0 +1,105 @@
+name: CI Checks (Rust fork)
+
+permissions:
+  contents: read
+
+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
+  push:
+    branches:
+      - 'rustc/**'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+jobs:
+  premerge-checks-linux:
+    name: Build and Test Linux
+    if: >-
+        github.repository_owner == 'rust-lang' &&
+        (github.event_name != 'pull_request' || github.event.action != 'closed')
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Checkout LLVM
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          fetch-depth: 1
+      - name: Free up disk space
+        run: |
+          curl -sSL https://raw.githubusercontent.com/rust-lang/rust/474466dc16f890083af1cdebad0aebf4d4ba88d7/src/ci/scripts/free-disk-space.sh | bash
+      - name: Install clang-18
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y clang-18 cmake ninja-build
+      - name: Setup ccache
+        uses: hendrikmuhs/ccache-action@a1209f81afb8c005c13b4296c32e363431bffea5 # v1.2.17
+        with:
+          max-size: "2000M"
+      - name: Build and Test
+        run: |
+          projects_to_build='llvm;lld'
+          project_check_targets='check-llvm check-lld'
+          runtimes_to_build=''
+          runtimes_check_targets=''
+          runtimes_check_targets_needs_reconfig=''
+
+          echo "Building projects: ${projects_to_build}"
+          echo "Running project checks targets: ${project_check_targets}"
+          echo "Building runtimes: ${runtimes_to_build}"
+          echo "Running runtimes checks targets: ${runtimes_check_targets}"
+          echo "Running runtimes checks requiring reconfiguring targets: ${runtimes_check_targets_needs_reconfig}"
+
+          export CC=/usr/bin/clang-18
+          export CXX=/usr/bin/clang++-18
+
+          ./.ci/monolithic-linux.sh "${projects_to_build}" "${project_check_targets}" "${runtimes_to_build}" "${runtimes_check_targets}" "${runtimes_check_targets_needs_reconfig}"
+
+  premerge-check-macos:
+    name: Build and Test macOS
+    runs-on: macos-14
+    if: >-
+      github.repository_owner == 'rust-lang' &&
+      (github.event_name != 'pull_request' || github.event.action != 'closed')
+    steps:
+      - name: Checkout LLVM
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          fetch-depth: 1
+      - name: Setup ccache
+        uses: hendrikmuhs/ccache-action@a1209f81afb8c005c13b4296c32e363431bffea5 # v1.2.17
+        with:
+          max-size: "2000M"
+      - name: Build and Test
+        run: |
+          projects_to_build='llvm'
+          project_check_targets='check-llvm'
+
+          echo "Building projects: ${projects_to_build}"
+          echo "Running project checks targets: ${project_check_targets}"
+
+          # -DLLVM_DISABLE_ASSEMBLY_FILES=ON is for
+          # https://github.com/llvm/llvm-project/issues/81967
+          # Disable sharding in lit so that the LIT_XFAIL environment var works.
+          cmake -G Ninja \
+                -B build \
+                -S llvm \
+                -DLLVM_ENABLE_PROJECTS="${projects_to_build}" \
+                -DLLVM_DISABLE_ASSEMBLY_FILES=ON \
+                -DCMAKE_BUILD_TYPE=Release \
+                -DLLDB_INCLUDE_TESTS=OFF \
+                -DLLVM_ENABLE_ASSERTIONS=ON \
+                -DCMAKE_C_COMPILER_LAUNCHER=ccache \
+                -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
+
+          # The libcxx tests fail, so we are skipping the runtime targets.
+          ninja -C build ${project_check_targets}