Start testing compiler-rt with llvm-next.

We want to test compiler-rt performance and stability.
So start testing llvm-next with compiler-rt as default using CL
513590.

BUG=chromium:705071
TEST=buildbot runs with CL 513590 are fine for daisy, squawks, elm and
peppy.

Change-Id: Ic43ef238c7fce0cb88caabab7e6a32b1940cb860
Reviewed-on: https://chromium-review.googlesource.com/514302


Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Trybot-Ready: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>