Properly check LLVM_FORCE_HEAD_REVISION

LLVM_FORCE_HEAD_REVISION is supposed to trigger building the latest
version of clang-cl when it is set to 1, but it was actually triggering
this when set to anything, including zero. Change the logic so that it
only triggers when set to non-zero. Setting to non-numeric will trigger
an exception.

Bug: 82385
Change-Id: Ib4ff79b1cfc61ea97545fa26f6d1f3f20086fc1f
Reviewed-on: https://chromium-review.googlesource.com/707674
Reviewed-by: smut <smut@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507823}
3 files changed