[VkShaderGen] Emit layout blocks for stage in/out

[ashwinbhat]

Update the Vulkan shadergen to emit layout blocks for stage interfaces.
old output:

layout (location = 0) out vec3 normalWorld;
layout (location = 1) out vec3 tangentWorld;
layout (location = 2) out vec3 positionWorld;

new output

layout (location = 0) out VertexData
{
    vec3 normalWorld;
    vec3 tangentWorld;
    vec3 positionWorld;
} vd;

Also fixes #2271

[linux-foundation-easycla]

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