[hlsl-out] Don't output interpolation modifier if it's the default

[NoelTautges]

Currently, hlsl-out writes the interpolation modifier before most structs, e.g.

struct input_0_ {
    linear float4 SV_Position : LOC0;
};

However, the linear intepolation modifier is the default in HLSL.

This pull request makes hlsl-output not output linear to make outputted code look more human-like.

[NoelTautges]

I forgot to test with --all-features. Let me change the output tests. 😅

[NoelTautges]

I ran the changed tests through FXC and the output was the same.