Optimize CI test workflow with caching

- Add ccache to speed up C++ compilation (2-10x faster rebuilds)
- Cache build directory between runs
- Cache key based on source file hashes (rebuilds only when code changes)
- Use LLVM_OPTIMIZED_TABLEGEN for faster initial builds
- First run: ~30-40 mins
- Subsequent runs with no changes: ~2-5 mins (just cache restore + test)
- Subsequent runs with small changes: ~5-10 mins (incremental build)

This makes the CI actually usable for regular development!

Author: cs01

.github/workflows/test-null-safety.yml

@@ -20,22 +20,42 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install -y ninja-build cmake build-essential
+          sudo apt-get install -y ninja-build cmake build-essential ccache
+
+      - name: Setup ccache
+        uses: hendrikmuhs/[email protected]
+        with:
+          key: ${{ github.job }}-${{ runner.os }}
+          max-size: 2G
+
+      - name: Cache build artifacts
+        uses: actions/cache@v4
+        with:
+          path: |
+            build
+          key: ${{ runner.os }}-clang-build-${{ hashFiles('llvm/**', 'clang/**') }}
+          restore-keys: |
+            ${{ runner.os }}-clang-build-
 
       - name: Configure build
         run: |
+          export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
           mkdir -p build
           cd build
           cmake -G Ninja \
             -DCMAKE_BUILD_TYPE=Release \
+            -DCMAKE_C_COMPILER_LAUNCHER=ccache \
+            -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
             -DLLVM_ENABLE_PROJECTS="clang" \
             -DLLVM_TARGETS_TO_BUILD="X86" \
             -DLLVM_ENABLE_ASSERTIONS=ON \
+            -DLLVM_OPTIMIZED_TABLEGEN=ON \
             -DLLVM_INCLUDE_TESTS=ON \
             ../llvm
 
       - name: Build Clang and test tools
         run: |
+          export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
           cd build
           ninja clang FileCheck count not