win: Re-enable Clang by default

Since the previous attempt there have been significant improvements in
debug info quality (both for debug builds and optimized builds which
affect minidumps) and link time.

Bug: 82385,709690
Change-Id: I0fef496d948855b38ce5ab93f66187798ac14b58
Reviewed-on: https://chromium-review.googlesource.com/732997
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510824}
7 files changed