Updating clang

1. Sync your Chromium tree to the latest revision to pick up any plugin changes
2. Run python tools/clang/scripts/upload_revision.py NNNN with the target LLVM SVN revision number. This creates a roll CL on a new branch, uploads it and starts tryjobs that build the compiler binaries into a staging bucket on Google Cloud Storage (GCS).
3. If the clang upload try bots succeed, copy the binaries from the staging bucket to the production one. For example:

$ export rev=123456-1
$ for x in Linux_x64 Mac Win ; do \
    gsutil cp -n -a public-read gs://chromium-browser-clang-staging/$x/clang-$rev.tgz \
        gs://chromium-browser-clang/$x/clang-$rev.tgz ; \
    gsutil cp -n -a public-read gs://chromium-browser-clang-staging/$x/llvmobjdump-$rev.tgz \
        gs://chromium-browser-clang/$x/llvmobjdump-$rev.tgz ; \
    gsutil cp -n -a public-read gs://chromium-browser-clang-staging/$x/translation_unit-$rev.tgz \
        gs://chromium-browser-clang/$x/translation_unit-$rev.tgz ; \
    done

1. Run the goma package update script to push these packages to goma. If you do not have the necessary credentials to do the upload, ask clang@chromium.org to find someone who does
2. Run an exhaustive set of try jobs to test the new compiler:

    git cl try &&
    git cl try -m tryserver.chromium.mac -b mac_chromium_asan_rel_ng &&
    git cl try -m tryserver.chromium.linux -b linux_chromium_chromeos_dbg_ng \
      -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
      -b fuchsia -b linux_chromium_cfi_rel_ng &&
    git cl try -m tryserver.blink -b linux_trusty_blink_rel

1. Commit roll CL from the first step
2. The bots will now pull the prebuilt binary, and goma will have a matching binary, too.