Turn on clang by default on linux.

tools/clang/scripts/update.sh

 #!/usr/bin/env bash
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script will check out llvm and clang into third_party/llvm and build it.
 
 # Do NOT CHANGE this if you don't know what you're doing -- see
 # https://code.google.com/p/chromium/wiki/UpdatingClang
 # Reverting problematic clang rolls is safe, though.
@@ skipping 125 matching lines @@
   if [[ "${OS}" == "Darwin" ]]; then
     # clang is used on Mac.
     true
   elif [[ "$GYP_DEFINES" =~ .*(clang|tsan|asan|lsan|msan)=1.* ]]; then
     # clang requested via $GYP_DEFINES.
     true
   elif [[ -d "${LLVM_BUILD_DIR}" ]]; then
     # clang previously downloaded, remove third_party/llvm-build to prevent
     # updating.
     true
+  elif [[ "${OS}" == "Linux" ]]; then
+    # Temporarily use clang on linux. Leave a stamp file behind, so that
+    # this script can remove clang again on machines where it was autoinstalled.
+    mkdir -p "${LLVM_BUILD_DIR}"
+    touch "${LLVM_BUILD_DIR}/autoinstall_stamp"
+    true
   else
     # clang wasn't needed, not doing anything.
     exit 0
   fi
 fi
 
 
 # Check if there's anything to be done, exit early if not.
 if [[ -f "${STAMP_FILE}" ]]; then
   PREVIOUSLY_BUILT_REVISON=$(cat "${STAMP_FILE}")
@@ skipping 331 matching lines @@
       "${TOOL_SRC_DIR}/tests/test.sh" "${LLVM_BUILD_DIR}/Release+Asserts"
     fi
   done
   pushd "${LLVM_BUILD_DIR}"
   ${MAKE} check-all
   popd
 fi
 
 # After everything is done, log success for this revision.
 echo "${CLANG_AND_PLUGINS_REVISION}" > "${STAMP_FILE}"