Vulkan: Use flat array lookups for shader variables. The array lookups we use after link now are simple array lookups and no longer use string hashing queries. This will make the descriptor cache indexing much faster after the cache key redesign. Bug: angleproject:4524 Change-Id: If19e3a4aa57c415f33c69172dd76c10a207e3264 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3580979 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>