clang: Let `gclient sync` autoupdate clang on linux if clang has been downloaded before.

tools/clang/scripts/update.sh

 #!/usr/bin/env bash
 # Copyright (c) 2011 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.
 
 THIS_DIR="$(dirname "${0}")"
 LLVM_DIR="${THIS_DIR}/../../../third_party/llvm"
 LLVM_BUILD_DIR="${LLVM_DIR}/../llvm-build"
@@ skipping 24 matching lines @@
       ;;
     --mac-only)
       mac_only=yes
       ;;
     --run-tests)
       run_tests=yes
       ;;
     --help)
       echo "usage: $0 [--force-local-build] [--mac-only] [--run-tests] "
       echo "--force-local-build: Don't try to download prebuilt binaries."
-      echo "--mac-only: Do nothing on non-Mac systems."
+      echo "--mac-only: Do initial download only on Mac systems."
       echo "--run-tests: Run tests after building. Only for local builds."
       exit 1
       ;;
   esac
   shift
 done
 
-if [[ -n "$mac_only" ]] && [[ "${OS}" != "Darwin" ]]; then
+# --mac-only prevents the initial download on non-mac systems, but if clang has
+# already been downloaded in the past, this script keeps it up to date even if
+# --mac-only is passed in and the system isn't a mac. People who don't like this
+# can just delete their third_party/llvm-build directory.
+if [[ -n "$mac_only" ]] && [[ "${OS}" != "Darwin" ]] &&
+    ! [[ -d "${LLVM_BUILD_DIR}" ]]; then
   exit 0
 fi
 
 # Xcode and clang don't get along when predictive compilation is enabled.
 # http://crbug.com/96315
 if [[ "${OS}" = "Darwin" ]] && xcodebuild -version | grep -q 'Xcode 3.2' ; then
   XCONF=com.apple.Xcode
   if [[ "${GYP_GENERATORS}" != "make" ]] && \
      [ "$(defaults read "${XCONF}" EnablePredictiveCompilation)" != "0" ]; then
     echo
@@ skipping 147 matching lines @@
 if [[ -n "$run_tests" ]]; then
   # Run a few tests.
   "${PLUGIN_SRC_DIR}/tests/test.sh" "${LLVM_BUILD_DIR}/Release+Asserts"
   cd "${LLVM_BUILD_DIR}"
   make check-all
   cd -
 fi
 
 # After everything is done, log success for this revision.
 echo "${CLANG_REVISION}" > "${STAMP_FILE}"