Prefer built-in system compilers over Clang download (#8355)

Co-authored-by: Mehrdad <noreply@github.com>

ci/travis/install-dependencies.sh

@@ -185,13 +185,15 @@ install_node() {
   fi
 }
 
+install_toolchains() {
+  "${ROOT_DIR}"/install-toolchains.sh
+}
+
 install_dependencies() {
 
   install_bazel
   install_base
-  if [ -n "${GITHUB_WORKFLOW-}" ]; then  # Not for Travis (keep built-in compilers there)
-    "${ROOT_DIR}"/install-toolchains.sh
-  fi
+  install_toolchains
   install_nvm
   install_pip
 

ci/travis/install-toolchains.sh

@@ -2,10 +2,10 @@
 
 set -euxo pipefail
 
-LLVM_VERSION="9.0.0"
+LLVM_VERSION="9.0.0"  # This is not necessarily guaranteed (e.g. we might use the system compiler)
 
-install_toolchains() {
-  local osversion="" url="" urlbase="https://releases.llvm.org" targetdir="/usr/local"
+install_clang() {
+  local cc="clang++" osversion="" url="" urlbase="https://releases.llvm.org" targetdir="/usr/local"
   case "${OSTYPE}" in
     msys)
       osversion=win
@@ -15,6 +15,7 @@ install_toolchains() {
         osversion="${osversion}32"
       fi
       url="${urlbase}/${LLVM_VERSION}/LLVM-${LLVM_VERSION}-${osversion}.exe"
+      cc="clang-cl"
       ;;
     linux-gnu)
       osversion="${OSTYPE}-$(sed -n -e '/^PRETTY_NAME/ { s/^[^=]*="\(.*\)"/\1/g; s/ /-/; s/\([0-9]*\.[0-9]*\)\.[0-9]*/\1/; s/ .*//; p }' /etc/os-release | tr '[:upper:]' '[:lower:]')"
@@ -26,18 +27,27 @@ install_toolchains() {
   if [ -z "${url}" ]; then
     url="${urlbase}/${LLVM_VERSION}/clang+llvm-${LLVM_VERSION}-${HOSTTYPE}-${osversion}.tar.xz"
   fi
-  curl -f -s -L -R "${url}" | if [ "${OSTYPE}" = "msys" ]; then
-    local target="./${url##*/}"
-    install /dev/stdin "${target}"
-    mkdir -p -- "${targetdir}"
-    7z x -bsp0 -bso0 "${target}" -o"${targetdir}"
-    MSYS2_ARG_CONV_EXCL="*" Reg Add "HKLM\SOFTWARE\LLVM\LLVM" /ve /t REG_SZ /f /reg:32 \
-      /d "$(cygpath -w -- "${targetdir}")" > /dev/null
-    rm -f -- "${target}"
-  else
-    sudo tar -x -J --strip-components=1 -C "${targetdir}"
+  if ! command -v "${cc}"; then
+    case "${osversion}" in
+      linux-gnu-ubuntu*)
+        sudo apt-get install -qq -o=Dpkg::Use-Pty=0 install clang clang-format clang-tidy
+        ;;
+      *)  # Fallback for all platforms is to download from LLVM's site, but avoided until necessary
+        local target="./${url##*/}"
+        curl -f -s -L -R --show-error -o "${target}" "${url}"
+        if [ "${OSTYPE}" = "msys" ]; then
+          mkdir -p -- "${targetdir}"
+          7z x -bsp0 -bso0 "${target}" -o"${targetdir}"
+          MSYS2_ARG_CONV_EXCL="*" Reg Add "HKLM\SOFTWARE\LLVM\LLVM" /ve /t REG_SZ /f /reg:32 \
+            /d "$(cygpath -w -- "${targetdir}")" > /dev/null
+          rm -f -- "${target}"
+        else
+          sudo tar -x -J --strip-components=1 -f "${target}" -C "${targetdir}"
+        fi
+        ;;
+    esac
   fi
-  "${targetdir}"/bin/clang --version 1>&2
+  "${cc}" --version
 }
 
-install_toolchains "$@"
+install_clang "$@"