WGSL: nointerpolate doesn't work

[aleino-nv]

Failing tests:

• tests/render/nointerpolation.hlsl (wgpu) is failing to generate the expected output (solid red triangle)

[aleino-nv]

Filing separate issue for tracking, but this will be done as part of #4943

[aleino-nv]

It should probably map to 'interpolate' attribute with https://www.w3.org/TR/WGSL/#interpolation-type-flat

[aleino-nv]

I'm working on a fix in the branch aleino/nointerpolate, but the resulting @interpolate attribute gets added to the CoarseVertex struct, whereas the actual output struct for the vertex shader (which also gets a @location attribute, as is required when using @interpolate) is the result of struct flattening.

I noticed that the struct flattening function does not move the interpolation mode decoration from the 'color' field over to the new flattened struct.

...

struct VertexStageOutput_0
{
    @location(0) color_0 : vec3<f32>,            // @interpolate *should* be here
    @builtin(position) sv_position_0 : vec4<f32>,
};

struct VertexStageInput_0
{
    @location(0) position_0 : vec3<f32>,
    @location(1) color_1 : vec3<f32>,
};

struct AssembledVertex_0
{
     position_1 : vec3<f32>,
     color_2 : vec3<f32>,
};

struct VertexStageInput_1
{
     assembledVertex_0 : AssembledVertex_0,
};

struct CoarseVertex_0
{
    @interpolate(flat)  color_3 : vec3<f32>,    // @interpolate *should not* be here
};

struct VertexStageOutput_1
{
     coarseVertex_0 : CoarseVertex_0,
     sv_position_1 : vec4<f32>,
};

@vertex
fn vertexMain( input_0 : VertexStageInput_0) -> VertexStageOutput_0
{
    ...
    var _S3 : VertexStageOutput_0;
    ...
    return _S3;
}

[aleino-nv]

PR based on the above observation #5641