Fragment shader integer built-ins incorrectly require `flat`

Integer Built-ins in fragment shaders currently require the flat decoration, like in our compiletest:
https://github.com/Rust-GPU/rust-gpu/blob/a30bd43db45f2bfe260051f44141e5eaffcbb4b0/tests/compiletests/ui/spirv-attr/all-builtins.rs#L96-L100

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.