This re-uses the same machinery as the GL front-end. It saves a lot of
custom casting and array sizing. Currently we only support vertex and
fragment shaders in Vulkan. Because of the Pipeline cache sizing, it's
easier to stick with just VS/FS and introduce the full set of shaders
when we move to packing the cache better.

Bug: angleproject:2522
Bug: angleproject:2455
Change-Id: I21432a335c741885af87970d8ee52b4a36338304
Reviewed-on: https://chromium-review.googlesource.com/1036927Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>