Casting integers to booleans generates a performance warning in at
least MSVS 2015 x64 Debug build. Specialize the clampCast template for
bool->int conversion to avoid this warning, which is treated as an
error.

BUG=angleproject:2165
TEST=MSVS 2015 x64 Debug build

Change-Id: Iaa9591c102cdd73fe9ff8a8739d356cc13cec248
Reviewed-on: https://chromium-review.googlesource.com/704820
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>