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/412266Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>