Vulkan: Add an extra descriptor set index In the case of CL, the clspv transcompiler can generate upto four descriptor set indices, so add an extra index to vk::DescriptorSetIndex. Also, adding aliases for CL specific naming. Bug: angleproject:369724757 Change-Id: I45ef8a6d9246c7863ebc6edf08479bc7c661c151 Signed-off-by: Gowtham Tammana <g.tammana@samsung.com> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5893953 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com>