Vulkan: Add a create shader helper. This will be useful for line segment rasterization. Bug: angleproject:2598 Change-Id: I84912c976665ecb32903181fa820b8db88786a28 Reviewed-on: https://chromium-review.googlesource.com/1127299 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>