Remove /fallback flag from 32-bit clang-cl invocations and enable -Werror

Clang can now compile all of Chromium in 32-bit as well as 64-bit without warnings.

R=thakis@chromium.org
BUG=82385

Review URL: https://codereview.chromium.org/1235033008

Cr-Commit-Position: refs/heads/master@{#339178}
2 files changed