[cbuildbot] Update toolchain waterfall builders.

Make the llvm-next waterfall builders use clang to build Chrome;
add more of the toolchain correctness tests to the toolchain builders.

BUG=None
TEST=Ran run_tests.

Change-Id: I2b53d54613037db1656d39a2e03b736b0bf5221d
Reviewed-on: https://chromium-review.googlesource.com/353654
Commit-Ready: Caroline Tice <cmtice@chromium.org>
Tested-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
2 files changed