workflows: Use sccache to speed up CI builds

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

lit-tests:
     steps:
       - name: Setup Windows
         if: startsWith(matrix.os, 'windows')
         uses: llvm/actions/setup-windows@main
         with:
           arch: amd64
       - name: Install Ninja
         uses: llvm/actions/install-ninja@main
+      # actions/checkout deletes any existing files in the new git directory,
+      # so this needs to either run before ccache-action or it has to use
+      # clean: false.
       - uses: actions/checkout@v3
         with:
           fetch-depth: 250
+      - name: Setup ccache
+        uses: hendrikmuhs/ccache-action@v1.2
+        with:
+          # A full build of llvm, clang, lld, and lldb takes about 250MB
+          # of ccache space. There's not much reason to have more than this,
+          # because we usually won't need to save cache entries from older
+          # builds.  Also, there is an overall 10GB cache limit, and each
+          # run creates a new cache entry so we want to ensure that we have
+          # enough cache space for all the tests to run at once and still
+          # fit under the 10 GB limit.
+          max-size: 500M
+          key: sccache-${{ matrix.os }}
+          variant: sccache
       - name: Build and Test
         uses: llvm/actions/build-test-llvm-project@main
         env:
           # Workaround for https://github.com/actions/virtual-environments/issues/5900.
           # This should be a no-op for non-mac OSes
           PKG_CONFIG_PATH: /usr/local/Homebrew/Library/Homebrew/os/mac/pkgconfig//11
         with:
-          cmake_args: '-GNinja -DLLVM_ENABLE_PROJECTS="${{ inputs.projects }}" -DCMAKE_BUILD_TYPE=Release -DLLDB_INCLUDE_TESTS=OFF'
+          cmake_args: '-GNinja -DLLVM_ENABLE_PROJECTS="${{ inputs.projects }}" -DCMAKE_BUILD_TYPE=Release -DLLDB_INCLUDE_TESTS=OFF -DCMAKE_C_COMPILER_LAUNCHER=sccache -DCMAKE_CXX_COMPILER_LAUNCHER=sccache'
           build_target:  '${{ inputs.build_target }}'
 
       - name: Build and Test libclc
         if: "!startsWith(matrix.os, 'windows') && contains(inputs.projects, 'libclc')"
         run: |
           # Make sure all of LLVM libraries that llvm-config needs are built.
           ninja -C build
           cmake -G Ninja -S libclc -B libclc-build -DLLVM_CONFIG=`pwd`/build/bin/llvm-config -DLIBCLC_TARGETS_TO_BUILD="amdgcn--;amdgcn--amdhsa;r600--;nvptx--;nvptx64--;nvptx--nvidiacl;nvptx64--nvidiacl"
           ninja -C libclc-build
           ninja -C libclc-build test