Clang update.sh: add support for LLVM_FORCE_HEAD_REVISION

Clang update.sh: add support for LLVM_FORCE_HEAD_REVISION

For the ToT builder(s).

BUG=452726
NOPRESUBMIT=true
NOTRY=true

Committed: https://crrev.com/f75504828f3e4bd059f89f18034a14c80a598885
Cr-Commit-Position: refs/heads/master@{#316287}

[hans, 2015-02-13 18:54:09]

hans@chromium.org changed reviewers:
+ thakis@chromium.org

[hans, 2015-02-13 18:54:09]

Please take a look.

[Nico, 2015-02-13 18:58:08]

lgtm, makes sense. Less bad than I had imagined :-)

[hans, 2015-02-13 20:37:36]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2015-02-13 20:38:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/923163002/1

[commit-bot: I haz the power, 2015-02-13 20:45:31]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-13 20:45:32]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[hans, 2015-02-13 20:55:03]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2015-02-13 20:55:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/923163002/1

[hans, 2015-02-13 21:02:54]

The CQ bit was unchecked by hans@chromium.org

[hans, 2015-02-13 21:03:06]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2015-02-13 21:03:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/923163002/1

[commit-bot: I haz the power, 2015-02-13 21:04:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-13 21:05:05]

Patchset 1 (id:??) landed as
https://crrev.com/f75504828f3e4bd059f89f18034a14c80a598885
Cr-Commit-Position: refs/heads/master@{#316287}