Roll Clang for Windows 201742:201859

BUG=none
TBR=rnk@chromium.org

Review URL: https://codereview.chromium.org/167953004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252603 0039d316-1c4b-4281-b951-d872f2087c98
Don't use automatic toolchain if GYP_MSVS_VERSION overridden to another version

This accomplishes three things:
- simple switch back to 2010 temporarily, if necessary
- making sure that all bots don't switch at exactly the same time (almost all have GYP_MSVS_VERSION=2010 in their settings)
- in particular, not having Official builders switch

Unfortunately, this will need to be merged on to the 34 branch, as it has the current "auto-use" code, so otherwise it'd switch to 2013 as soon as depot_tools starts updating the toolchain (previously, the only check was whether the toolchain exists on the local hard drive).

R=maruel@chromium.org
BUG=323300

Review URL: https://codereview.chromium.org/174143003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252604 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252604