gecko-dev/build/build-clang/skip-3-stages.json

{
    "skip_stages": "3",
    "cc": "{MOZ_FETCHES_DIR}/clang/bin/clang",
    "cxx": "{MOZ_FETCHES_DIR}/clang/bin/clang++",
    "as": "{MOZ_FETCHES_DIR}/clang/bin/clang"
}