[libc++][ci] Use ccache in the jobs that build Clang

This is an attempt to reduce the time taken by the Bootstrapping
build job and the Clang CI job that builds the compiler from scratch.

Differential Revision: https://reviews.llvm.org/D150908

libcxx/utils/ci/buildkite-pipeline-clang.yml

@@ -23,8 +23,9 @@ steps:
       # We use Release here to avoid including debug information. Otherwise, the clang binary is very large, which
       # is problematic because we need to upload the artifacts for other jobs to use. This may seem like nothing,
       # but with the number of jobs we run daily, this can result in thousands of GB of network I/O.
-      - "cmake -S llvm -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install -DLLVM_ENABLE_PROJECTS=\"clang;compiler-rt\""
+      - "cmake -S llvm -B build -G Ninja -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install -DLLVM_ENABLE_PROJECTS=\"clang;compiler-rt\""
       - "ninja -C build install-clang install-clang-resource-headers"
+      - "ccache -s"
       - "tar -cJvf install.tar.xz install/"
       - "buildkite-agent artifact upload --debug install.tar.xz"
     env:

libcxx/utils/ci/run-buildbot

@@ -340,6 +340,7 @@ bootstrapping-build)
           -S "${MONOREPO_ROOT}/llvm" \
           -B "${BUILD_DIR}" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
+          -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang" \