[[vk::push_constant]] variables are emitted as uniforms in SPIR-V

I am using HLSL for a project that uses vulkan and when trying to make use of push constants in a compute shader I noticed that the emitted SPIR-V code is incorrect.

Instead of annotating the push constant data as such it is marked as `Uniform`, which makes the creation of a compute pipeline fail.

The corresponding GLSL code emits the correct SPIR-V code.

HLSL code (not working) playground

```hlsl
struct PushData {
float val;
};

[[vk::push_constant]]
PushData pushData;

[[vk::binding(0)]]
RWStructuredBuffer data;

void CSMain()
{
data[0] = pushData.val;
}
```

GLSL code (working) playground

```glsl
#version 450

layout(push_constant) uniform PushData {
float val;
} push_data;

layout(binding = 0) buffer Output {
float data[];
};

void main() {
data[0] = push_data.val;
}

```

Run the linked HLSL playground example and compare its generated SPIR-V with the linked GLSL example. Trace the HLSL-to-SPIR-V handling of [[vk::push_constant]] and verify that the emitted push-constant data is annotated correctly rather than as Uniform, so compute pipeline creation succeeds.

Written by the indexing model from the issue text.