Clean up output Vertices and Indices types for mesh shader.

[csyonghe]

In our mesh shader representation, we currently use out Vertices<T> and out Indices<T> type to represent output vertex and index arrays.

This representation isn't aligned with the semantics of out parameters. In this case, a Vertex<T> type represents a reference to a output buffer, and the reference itself shouldn't be an out. Instead, we should introduce OutputVertices<T> type and turn the parameter into just OutputVertices<T> type, similar to how OutputPatch type works.

The current representation caused an issue in the uninitialized out parameter checking pass (slang-ir-use-uninitialized-out-param.cppbecause we will generate code that firstloadthe reference from theout` parameter, and then write to the reference, which isn't correct in terms of types.

After we clean this up, we should be able to remove the temporary hack that skips checking these parameters in the slang-ir-use-uninitialized-out-param pass.

[natduca]

From Yong, I came to understand that the situation here is that the shape we have now is based on compat with HLSL, which we want to keep. But ideally we need a parser stage solution that takes "hlsl dialect" input and lowers it into our internal "ideal" approach described above.

We would however, in that solution, maintain the external suport for hlsl's style. So we don't need to do this right way.

The reason we want to still do this is that we're misusing the type system, and that does cause us some problems long term. So we want to do the change.

With this said, language cleanup, nonbreaking once we do it, so Q3.