roll clang 254049:254793

This switches msan to the new ABI.

It's also the first roll that used update.py instead of
update.sh to build the packages.

It also changes package.py to bundle up libstdc++ on linux
unconditionally, and removes the --gcc-toolchain flag from
it (the auto-downloaded gcc should be good enough;
one fewer thing I can get wrong).

BUG=565541

Review URL: https://codereview.chromium.org/1507873002

Cr-Commit-Position: refs/heads/master@{#363986}
8 files changed