BUG=angle:736

va_start behavior is undefined if the second parameter is a
function, array, or reference type. clang produces a warning
for this, while MSVC does not.

Change-Id: I0bc2805e312e3542aac816f10a257e2f1cfad128
Reviewed-on: https://chromium-review.googlesource.com/216010Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Tested-by: Nico Weber <thakis@chromium.org>