Add nightly build with the latest LLVM

.github/workflows/nightly-build.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: CI
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  push:
+    branches: [ main ]
+  schedule:
+    # 20:00 JST
+    - cron: "0 11 * * *"
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "build"
+  build:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+      - uses: actions/checkout@v2
+      - uses: seanmiddleditch/gha-setup-ninja@master
+
+      - name: Install prerequisites
+        run: |
+          sudo apt update
+          sudo apt install -y uuid-dev
+
+      - name: Cache LLVM artifact
+        id: cache-llvm
+        uses: actions/cache@v2
+        with:
+          path: |
+            ./thirdparty/llvm-project
+          key: ${{ runner.os }}-norm-${{ hashFiles('thirdparty/llvm-project/llvm/CMakeLists.txt') }}
+
+      - name: Build with the latest LLVM
+        if: steps.cache-llvm.outputs.cache-hit != 'true'
+        run: |
+          ./update-llvm.sh