Vulkan: Create/use a constant for the highest Vulkan version used. Bug: angleproject:3036 Change-Id: Ica056743eed2aa08e96b8a85e693ae70355b52d5 Reviewed-on: https://chromium-review.googlesource.com/c/1464739 Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Ian Elliott <ianelliott@google.com> Commit-Queue: Ian Elliott <ianelliott@google.com>