Use sccache to speed up verify build.

.github/workflows/verify.yml

@@ -20,10 +20,23 @@ jobs:
         with:
           repository: 'llvm-mos/llvm-mos'
           path: 'llvm-mos'
+      - 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 the compiler.
         run: |
           cd llvm-mos
-          cmake -C clang/cmake/caches/MOS.cmake -G Ninja -S llvm -B build -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_USE_LINKER=lld
+          cmake -C clang/cmake/caches/MOS.cmake -G Ninja -S llvm -B build -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_USE_LINKER=lld -DCMAKE_C_COMPILER_LAUNCHER=sccache -DCMAKE_CXX_COMPILER_LAUNCHER=sccache
           cd build
           cmake --build . --target clang lld llvm-ar llvm-ranlib llvm-size llvm-mlb
       - name: Archive the compiler.