vkd3d-shader: Introduce VKD3D_SHADER_TARGET_MSL.
vkd3d-shader/msl: Generate comments for unhandled instructions. Similar to commit 8a60814476b1b05ef72ae78e9ded80d4c03c3b1a for the GLSL target.
vkd3d-shader/msl: Call vsir_program_transform() before generating code.