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

This way, clang is autoupdated for people who use clang on linux, but people and bots
who don't use it are still not affected

BUG=none
TEST=linux clang users stop complaining to me about things that were fixed long ago.


Review URL: http://codereview.chromium.org/8429020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@108118 0039d316-1c4b-4281-b951-d872f2087c98

tools/clang/scripts/update.sh

@@ -42,7 +42,7 @@ while [[ $# > 0 ]]; do
     --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
       ;;
@@ -50,7 +50,12 @@ while [[ $# > 0 ]]; do
   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