Rolling v8/tools/clang to
b6f620b311665e2d96d0921833f54295b9bbf925

Porting parts of https://codereview.chromium.org/1917853002

BUG=604993
LOG=n
TBR=thakis@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/1923473002

Cr-Commit-Position: refs/heads/master@{#35782}