clang/win/asan: Remove /fallback and enable warnings-as-errors.

clang/win/asan: Remove /fallback and enable warnings-as-errors.

This removes the last case where we still needed /fallback.

BUG=82385
Committed: https://crrev.com/0fa9d4fad367939eb0ac68b86914c6d86339da3c
Cr-Commit-Position: refs/heads/master@{#340533}

[Nico, 2015-07-24 23:49:54]

thakis@chromium.org changed reviewers:
+ rnk@chromium.org

[Nico, 2015-07-27 18:16:57]

rnk: ping :-)

[Reid Kleckner, 2015-07-27 18:21:56]

lgtm

Speaking of which, I should close that PR...

[Reid Kleckner, 2015-07-27 18:21:59]

The CQ bit was checked by rnk@chromium.org

[commit-bot: I haz the power, 2015-07-27 18:22:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1259813002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1259813002/1

[commit-bot: I haz the power, 2015-07-27 19:43:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-27 19:44:51]

Patchset 1 (id:??) landed as
https://crrev.com/0fa9d4fad367939eb0ac68b86914c6d86339da3c
Cr-Commit-Position: refs/heads/master@{#340533}