Use Zircon's Clang rather than building our own

This toolchain is already used to build Zircon itself and is the
official Clang toolchain used by all Fuchsia developers.

src/ci/docker/dist-fuchsia/Dockerfile

@@ -3,7 +3,6 @@ FROM ubuntu:16.04
 RUN apt-get update && apt-get install -y --no-install-recommends \
   g++ \
   make \
-  ninja-build \
   file \
   curl \
   ca-certificates \
@@ -11,16 +10,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   git \
   sudo \
   bzip2 \
-  xz-utils \
-  swig \
-  libedit-dev \
-  libncurses5-dev \
-  patch \
   unzip
 
-RUN curl -L https://cmake.org/files/v3.9/cmake-3.9.2-Linux-x86_64.tar.gz | \
-      tar xzf - -C /usr/local --strip-components=1
-
 WORKDIR /tmp
 COPY dist-fuchsia/shared.sh dist-fuchsia/build-toolchain.sh /tmp/
 RUN /tmp/build-toolchain.sh

src/ci/docker/dist-fuchsia/build-toolchain.sh

@@ -14,38 +14,22 @@
 set -ex
 source shared.sh
 
-# Download sources
-SRCS=(
-  "https://fuchsia.googlesource.com/zircon zircon e9a26dbc70d631029f8ee9763103910b7e3a2fe1"
-  "https://llvm.googlesource.com/llvm llvm 65bdf0ae4a87e6992c24f06e2612909952468710"
-  "https://llvm.googlesource.com/clang llvm/tools/clang 914987de45cf83636537909ce09156aa7a37d6ec"
-  "https://llvm.googlesource.com/clang-tools-extra llvm/tools/clang/tools/extra 83de24124250a7cdc7a0fdc61b7e3c3d64b80225"
-  "https://llvm.googlesource.com/lld llvm/tools/lld f8ed4483c589b390daafac92e28f4680ad052643"
-  "https://llvm.googlesource.com/lldb llvm/tools/lldb 55cf8753321782668cb7e2d879457ee1ad57a2b9"
-  "https://llvm.googlesource.com/compiler-rt llvm/runtimes/compiler-rt a8682fdf74d3cb93769b7394f2cdffc5cefb8bd8"
-  "https://llvm.googlesource.com/libcxx llvm/runtimes/libcxx 5f919fe349450b3da0e29611ae37f6a940179290"
-  "https://llvm.googlesource.com/libcxxabi llvm/runtimes/libcxxabi caa78daf9285dada17e3e6b8aebcf7d128427f83"
-  "https://llvm.googlesource.com/libunwind llvm/runtimes/libunwind 469bacd2ea64679c15bb4d86adf000f2f2c27328"
-)
+ZIRCON=e9a26dbc70d631029f8ee9763103910b7e3a2fe1
 
-fetch() {
-  mkdir -p $2
-  pushd $2 > /dev/null
-  git init
-  git remote add origin $1
-  git fetch --depth=1 origin $3
-  git reset --hard FETCH_HEAD
-  popd > /dev/null
-}
+mkdir -p zircon
+pushd zircon > /dev/null
 
-for i in "${SRCS[@]}"; do
-  fetch $i
-done
+# Download sources
+git init
+git remote add origin https://fuchsia.googlesource.com/zircon
+git fetch --depth=1 origin $ZIRCON
+git reset --hard FETCH_HEAD
 
-# Build sysroot
-./zircon/scripts/download-toolchain
+# Download toolchain
+./scripts/download-toolchain
+cp -a prebuilt/downloads/clang+llvm-x86_64-linux/. /usr/local
 
-build_sysroot() {
+build() {
   local arch="$1"
 
   case "${arch}" in
@@ -53,33 +37,20 @@ build_sysroot() {
     aarch64) tgt="zircon-qemu-arm64" ;;
   esac
 
-  hide_output make -C zircon -j$(getconf _NPROCESSORS_ONLN) $tgt
+  hide_output make -j$(getconf _NPROCESSORS_ONLN) $tgt
   dst=/usr/local/${arch}-unknown-fuchsia
   mkdir -p $dst
-  cp -r zircon/build-${tgt}/sysroot/include $dst/
-  cp -r zircon/build-${tgt}/sysroot/lib $dst/
+  cp -a build-${tgt}/sysroot/include $dst/
+  cp -a build-${tgt}/sysroot/lib $dst/
 }
 
+# Build sysroot
 for arch in x86_64 aarch64; do
-  build_sysroot ${arch}
+  build ${arch}
 done
 
-# Build toolchain
-cd llvm
-mkdir build
-cd build
-hide_output cmake -GNinja \
-  -DFUCHSIA_x86_64_SYSROOT=/usr/local/x86_64-unknown-fuchsia \
-  -DFUCHSIA_aarch64_SYSROOT=/usr/local/aarch64-unknown-fuchsia \
-  -DLLVM_ENABLE_LTO=OFF \
-  -DCLANG_BOOTSTRAP_PASSTHROUGH=LLVM_ENABLE_LTO \
-  -C ../tools/clang/cmake/caches/Fuchsia.cmake \
-  ..
-hide_output ninja stage2-distribution
-hide_output ninja stage2-install-distribution
-cd ../..
-
-rm -rf zircon llvm
+popd > /dev/null
+rm -rf zircon
 
 for arch in x86_64 aarch64; do
   for tool in clang clang++; do