Some fixes to valid clangd warnings, since I am using it as linter.
Also, I am removing enum hlsl_error_level on 4/6, since it doesn't seem to be meant to be used anywhere after 3/6.
-- v2: vkd3d-shader/d3dbc: Use D3DSIO_TEXKILL instead of VKD3D_SM1_OP_TEXKILL (clangd). vkd3d-shader/hlsl: Remove enum hlsl_error_level (clangd). vkd3d-shader/tpf: Avoid translations to D3DDECLUSAGE and back (clangd). vkd3d-shader: Remove unnecessary fallthroughs (clangd).