Reject @location in non-entry-point function argments

[nical]

Naga currently accepts:

// Not an entry point, but takes an argument with a location.
fn foo(@location(1) bar: vec3<u32>) -> vec3<u32> {
    return bar;
}

@vertex
fn vs_main(
    @location(0) position: vec2<i32>,
) -> @builtin(position) vec4<f32> {
    var v = foo();

    // ...
}

However https://gpuweb.github.io/gpuweb/wgsl/#attribute-location says: "Must only be applied to an entry point function parameter, entry point return type, or member of a structure type."