[HLSL][SPIRV] Implement fragment shaders

[s-perron]

Add the ability to compile a passthrough fragment (pixel) shader from end to end from HLSL targeting SPIR-V.

This will be considered done when the following shader can be compiled using clang (https://godbolt.org/z/xTK87KhEE):

// Compile:  clang-dxc t.hlsl -T ps_6_8 -E main -spirv -fspv-target-env=vulkan1.3
void main() {}