attr: fix #[spirv(flat)] checking to match the Vulkan spec.

[eddyb]

Rust-GPU validation for #[spirv(flat)] was added in #815 but it had both:

• false positives: requiring it based on type alone, even in places not even allowed by the Vulkan spec
• false negatives: not requiring on builtin fragment inputs (because of an unrelated early-return for bools)

The Vulkan spec validation rules relevant here are:

• VUID-StandaloneSpirv-Flat-04670

  The Flat, NoPerspective, Sample, and Centroid decorations must only be used on variables with the Output or Input storage class

• VUID-StandaloneSpirv-Flat-04744

  Any variable with integer or double-precision floating-point type and with Input storage class in a fragment shader, must be decorated Flat

• VUID-StandaloneSpirv-Flat-06201

  The Flat, NoPerspective, Sample, and Centroid decorations must not be used on variables with the Output storage class in a fragment shader

• VUID-StandaloneSpirv-Flat-06202

  The Flat, NoPerspective, Sample, and Centroid decorations must not be used on variables with the Input storage class in a vertex shader

This was prompted by spirv-val improving their own checks (blocking updating SPIRV-Tools in #928):

• spirv-val: Add Vulkan decoration interface KhronosGroup/SPIRV-Tools#4831
• spirv-val: Improve PR 4831 error message KhronosGroup/SPIRV-Tools#4968