Support for combinedImageSamplers?

The microsoft compiler supports a special atribute for compatibility with glsl:
https://github.com/microsoft/DirectXShaderCompiler/wiki/Vulkan-combined-image-sampler-type

However declaring this in an hlsl shader:
```
[[vk::combinedImageSampler]]
Texture2D textures[3] : register(t1);
```

And compiling with shaderc results in the error:
`examples/02_skinned_mesh/test.frag.hlsl:1: warning: 'combinedImageSampler' : unrecognized attribute`

It would be a nice addition to shaderc I think. I would certainly find it very useful.