Add support for downloading clang

Add support for downloading clang

Still requires changes to gyp files to select the built-in clang as
compiler and make clang the default on the platforms we want it.

BUG=
R=dcarney@chromium.org,machenbach@chromium.org
LOG=n

[dcarney, 2014-11-26 14:18:52]

lgtm

[Michael Achenbach, 2014-11-26 15:08:34]

lgtm

https://codereview.chromium.org/745963005/diff/1/DEPS
File DEPS (right):

https://codereview.chromium.org/745963005/diff/1/DEPS#newcode22
DEPS:22: Var("git_url") +
"/chromium/src/tools/clang.git@327e26ee7f6ae6f86fb335113b46cb86ed3cb0dc",
Be aware of make dependencies...

https://codereview.chromium.org/745963005/diff/1/DEPS#newcode87
DEPS:87: # Pull clang if needed or requested via GYP_DEFINES.
I don't understand how this is dependent on GYP_DEFINES. Is the update script
only called if clang=1 or always?

[jochen (gone - plz use gerrit), 2014-11-27 10:01:39]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-11-27 10:02:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/745963005/1

[jochen (gone - plz use gerrit), 2014-11-27 10:02:39]

https://codereview.chromium.org/745963005/diff/1/DEPS
File DEPS (right):

https://codereview.chromium.org/745963005/diff/1/DEPS#newcode22
DEPS:22: Var("git_url") +
"/chromium/src/tools/clang.git@327e26ee7f6ae6f86fb335113b46cb86ed3cb0dc",
On 2014/11/26 15:08:34, Michael Achenbach wrote:
> Be aware of make dependencies...

everybody should be using gclient sync, no?

https://codereview.chromium.org/745963005/diff/1/DEPS#newcode87
DEPS:87: # Pull clang if needed or requested via GYP_DEFINES.
On 2014/11/26 15:08:34, Michael Achenbach wrote:
> I don't understand how this is dependent on GYP_DEFINES. Is the update script
> only called if clang=1 or always?

it internally checks the gyp defines

[Michael Achenbach, 2014-11-27 10:13:05]

https://codereview.chromium.org/745963005/diff/1/DEPS
File DEPS (right):

https://codereview.chromium.org/745963005/diff/1/DEPS#newcode22
DEPS:22: Var("git_url") +
"/chromium/src/tools/clang.git@327e26ee7f6ae6f86fb335113b46cb86ed3cb0dc",
On 2014/11/27 10:02:39, jochen (slow) wrote:
> On 2014/11/26 15:08:34, Michael Achenbach wrote:
> > Be aware of make dependencies...
> 
> everybody should be using gclient sync, no?

I can't speak for other people. The best way to enforce that is to remove the
dependencies target from the Makefile. I'll do that and wait for complaints...

[commit-bot: I haz the power, 2014-11-27 10:29:46]

Committed patchset #1 (id:1)