[wgsl in and IR] naga ray tracing pipelines

[Vecvec]

Connections
Initial proposal for the wgsl front-end of #6760

Description
Adds wgsl-in and (partial) validation for the simplest version of ray tracing pipelines. This matches the spirv and hlsl specs to the best of my knowledge (though with less than complete validation). Things may change in the future though. @JMS55 may wish to check the naming ect. I don't normally open a naga PR w/o a full way through, but last time I tried pipelines I did that and struggled to keep it up to date.

Naming differences from vulkan
LaunchId -> ray_invocation_id (I think its clearer what it is talking about)
LaunchSize -> num_ray_invocations
InstanceCustomIndex -> instance_custom_data (like ray queries)
RayTmax -> ray_t_current_max (unlike RayTmin, it changes over the life cycle of the ray, and doesn't match the field of the ray desc)

Testing
Snapshot test from wgsl to IR.

Squash or Rebase?
Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests.
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[JMS55]

Very exciting!! I will take a look sometime this week for sure!

[cwfitzgerald]

Looking good! Some small things then lets land and iterate, the tests look nice.

[Vecvec]

Just going to note that spv-out is being worked on. Right now built-ins aren't yet supported, as I wanted to first implement traceRay (which is the most different part compared to everything else, the built-ins should just be mapping really).

[JMS55]

Lgtm in terms of RT pipelines 1.0/1.1, but we should keep in mind future support for SER hit objects (DX: https://github.com/microsoft/DirectX-Specs/blob/master/d3d/Raytracing.md#hitobject. VK: https://www.khronos.org/blog/boosting-ray-tracing-performance-with-shader-execution-reordering-introducing-vk-ext-ray-tracing-invocation-reorder).

The main draw of RT pipelines for me is SER, so we should try and keep the API similar/transparent if we can.

[Vecvec]

The main draw of RT pipelines for me is SER, so we should try and keep the API similar/transparent if we can.

The API can't be transparent (without making many concessions), but changes for it should be limited to the trace ray calls (which even now reorder, just without any hints).