Revert of Use third_party clang on Mac. (patchset #4 id:60001 of https://codereview.chromium.org/1200833013/)

Reason for revert:
[Sheriff] Breaks mac on the main waterfall (though not on the tryserver):
http://build.chromium.org/p/client.v8/builders/V8%20Mac64/builds/3925

Original issue's description:
> Use third_party clang on Mac.
>
> BUG=
>
> Committed: https://crrev.com/57b20413294940476ab2b2e71ed0802aaf8c223f
> Cr-Commit-Position: refs/heads/master@{#29343}

TBR=danno@chromium.org,jkummerow@chromium.org,paul.lind@imgtec.com,akos.palfi@imgtec.com,balazs.kilvady@imgtec.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1217823003

Cr-Commit-Position: refs/heads/master@{#29345}