Vulkan: Implement new GLSL translator back-end. The Vulkan GLSL translator back-end will handle validating and translating our WebGL/ESSL shaders into Vulkan-specific GLSL. glslang (the Vulkan one) accepts both GLSL and GLSL ES shaders as inputs, and both the desktop and ESSL back-ends give incompleteness warnings when used. For now, use the desktop GL 450 as a target for Vulkan GLSL. The Vulkan-specific changes are currently only to add locations to every vertex input and fragment output. BUG=angleproject:1575 Change-Id: I7c3f32f522e9d18e5f8618eb7927336bf4fbdcf2 Reviewed-on: https://chromium-review.googlesource.com/412266 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>