clang: Disable -instcombine-lower-dbg-declare=1 for CrOS builds

It breaks CrOS builds which use a different version of Clang, which
doesn't support this flag.

This is a stop-gap fix; Linux CrOS builds do use the Chromium version of
Clang, and we want them to have this flag, but there doesn't seem to be
a straight-forward way to distinguish between these compiler versions
from the build system.

Landing to unbreak the build for now until a better solution is found.

TBR=llozano

Bug: 766311,765793
Change-Id: I3e040161f6987b502b28e97f41fcde2d464da72c
Reviewed-on: https://chromium-review.googlesource.com/671810
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502668}
1 file changed