clang: Instead of having update.sh clobber out/, add a define with the current revision.

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 60 matching lines @@
   case $1 in
     --bootstrap)
       bootstrap=yes
       ;;
     --if-needed)
       if_needed=yes
       ;;
     --force-local-build)
       force_local_build=yes
       ;;
+    --print-revision)
+      echo $CLANG_REVISION
+      exit 0
+      ;;
     --run-tests)
       run_tests=yes
       ;;
     --without-android)
       with_android=
       ;;
     --with-chrome-tools)
       shift
       if [[ $# == 0 ]]; then
         echo "--with-chrome-tools requires an argument."
@@ skipping 15 matching lines @@
         exit 1
       fi
       ;;
 
     --help)
       echo "usage: $0 [--force-local-build] [--if-needed] [--run-tests] "
       echo "--bootstrap: First build clang with CC, then with itself."
       echo "--force-local-build: Don't try to download prebuilt binaries."
       echo "--if-needed: Download clang only if the script thinks it is needed."
       echo "--run-tests: Run tests after building. Only for local builds."
+      echo "--print-revision: Print current clang revision and exit."
       echo "--without-android: Don't build ASan Android runtime library."
       echo "--with-chrome-tools: Select which chrome tools to build." \
            "Defaults to plugins."
       echo "    Example: --with-chrome-tools 'plugins empty-string'"
       echo "--gcc-toolchain: Set the prefix for which GCC version should"
       echo "    be used for building. For example, to use gcc in"
       echo "    /opt/foo/bin/gcc, use '--gcc-toolchain '/opt/foo"
       echo
       exit 1
       ;;
@@ skipping 38 matching lines @@
        [[ "${PREVIOUSLY_BUILT_REVISON}" = \
           "${CLANG_AND_PLUGINS_REVISION}" ]]; then
     echo "Clang already at ${CLANG_AND_PLUGINS_REVISION}"
     exit 0
   fi
 fi
 # To always force a new build if someone interrupts their build half way.
 rm -f "${STAMP_FILE}"
 
 
-# Clobber all output files. PCH files only work with the compiler that created
-# them, so we need clobber the output files to make sure they are rebuilt
-# using the new compiler.
-echo "Clobbering build files"
-MAKE_DIR="${THIS_DIR}/../../../out"
-XCODEBUILD_DIR="${THIS_DIR}/../../../xcodebuild"
-for DIR in "${XCODEBUILD_DIR}" "${MAKE_DIR}"; do
-  if [[ -d "${DIR}" ]]; then
-    rm -rf "${DIR}"
-  fi
-done
-
 if [[ -z "$force_local_build" ]]; then
   # Check if there's a prebuilt binary and if so just fetch that. That's faster,
   # and goma relies on having matching binary hashes on client and server too.
   CDS_URL=https://commondatastorage.googleapis.com/chromium-browser-clang
   CDS_FILE="clang-${CLANG_REVISION}.tgz"
   CDS_OUT_DIR=$(mktemp -d -t clang_download.XXXXXX)
   CDS_OUTPUT="${CDS_OUT_DIR}/${CDS_FILE}"
   if [ "${OS}" = "Linux" ]; then
     CDS_FULL_URL="${CDS_URL}/Linux_x64/${CDS_FILE}"
   elif [ "${OS}" = "Darwin" ]; then
@@ skipping 288 matching lines @@
       "${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}"