Update Vulkan ray tracing support to final extension spec (#717)

* Update version of glslang used

* Update VK raytracing support for final extension spec

A lot of this change is just plain renaming: The `NVX` suffixes become just `NV`, and the extension name changes from `GL_NVX_raytracing` to `GL_NV_ray_tracing`.
The Slang standard library and the GLSL baselines for the tests are consistently updated.

The other detail is that the final spec requires the "payload" identifier in a `traceNV()` call to be a compile-time constant, which means it cannot be defined as a local variable first, as in:

```glsl
int payloadID = 0;
traceNV(..., payloadID); // ERROR
```

In terms of how the original support was implemented, the payload ID is being computed via a special builtin function that maps each global GLSL payload variable to a unique ID. There are a few ways we could try to resolve the problem here:

1. We could aspire to put our equivalent of the `constexpr` modifier on the output of the function, so that the GLSL variable gets declared `const` and thus fits the GLSL rules for a constant expression.

2. We could introduce a pass to replace the payload-location instructions with literal integers.

3. We could use a special-purpose instruction instead of a builtin function call, and have that instruction indicate that it doesn't have side effects (so it can be folded into the call site)

4. We could somehow mark the builtin function as not having side effects.

We choose option (4) simply because it provides a feature that could have other applications. This change adds a `[__readNone]` attribute that can be applied to function declarations to express a promise on the part of the programmer that the given function has no side effects and computes its result strictly from the bits of its input arguments (and not things they point to, etc.). This mirrors an equivalent function attribute in LLVM.

We mark the function that computes a ray payload location with this attribute, and propagate the attribute through the layers of the IR, so that when the emit logic asks if an operation has side effects (to see if it can be folded into the arguments of a subsequent expression), we get an affirmative response.

This change should get all of the features that were present in the experiemntal `NVX` extension working with the final extension spec. It does not address callable shaders, which will come as a subsequent change.

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