clang tools: Add --bootstrap to the documentation for update.py

This lets us build the clang tools with clang not gcc, which appears to
have bugs!

Also work around the gcc bug in one place we know it happens anyways,
just because.

R=dcheng
BUG=580745

Review URL: https://codereview.chromium.org/1647733002

Cr-Commit-Position: refs/heads/master@{#371933}
2 files changed