valid: Check regular functions don't have bindings

[JCapucho]

Checks that all arguments and return types have no bindings in regular functions.

Closes #2037

[jimblandy]

Interesting. I thought this program would be forbidden by WGSL, but the spec doesn't seem to say it is:

struct Foo {
   @location(0) x: f32,
};

fn f(arg: Foo) -> f32 {
   return arg.x;
}

Tint accepts the program.

[jimblandy]

Thank you! This looks good, but the error messages need to be clarified:

[JCapucho]

Interesting. I thought this program would be forbidden by WGSL, but the spec doesn't seem to say it is:

struct Foo {
   @location(0) x: f32,
};

fn f(arg: Foo) -> f32 {
   return arg.x;
}

Tint accepts the program.

I guess this is to allow for programs like:

struct Foo {
   @location(0) x: f32,
};

@vertex
fn vs_main(foo: Foo) -> @location(0) vec4<f32> {
  let a = f(foo);
  return vec4<f32>(a);
}

fn f(arg: Foo) -> f32 {
    return arg.x;
}

[jimblandy]

Fantastic - thank you very much!