Fragment shader integer built-ins incorrectly require flat #503

Integer Built-ins in fragment shaders currently require the flat decoration, like in our compiletest:

otherwise compile will fail with:

error: `Fragment` entry-point `Input` parameter must be decorated with `#[spirv(flat)]`
    --> crates/restir-shader/src/visibility/raster.rs:40:39
     |
  40 |     #[spirv(primitive_id)] primitive_id: u32,
     |                                          ^^^

However, decorating them with flat makes no sense and may even be illegal, since they are builtins, and flat configures interpolation behaviour.