clang/win: Add back /fallback for asan builds for now.

In asan builds, a few translation units still fall back to CL, see
http://llvm.org/PR24167

Also update the gn files to pass /WX (warnings-as-errors) in 32-bit non-asan
clang builds.

BUG=82385
TBR=rnk

Review URL: https://codereview.chromium.org/1229083005.

Cr-Commit-Position: refs/heads/master@{#339309}
2 files changed