Filter completion items based on access modifiers (e.g. public/private)

[MartTave]

For now, the completion serves all the members of a struct without any distinction for the access modifier.

As a first step, we could at least differentiate the two when serving completion items.

But in its final state, we might need to detect the current scope and accessibility of the members, and put a warning (or something else) to tell the user that this member exists, but is not accessible in this context.

Example :

struct A {
  public memberwise init
  private let m1: Bool
}

let a = A(m1: true)

a. // <- trigger completion here
// For now, it returns :
// - m1: let Bool

// And it should return something like :
// - private m1: Bool
// or :
// - m1: Bool          ! This member is not accessible in this scope

I'm not convinced of the solutions I'm proposing, but I'm pretty confident that we can't leave it as is :)

[tothambrus11]

It seems access modifiers are not yet checked in the frontend, so it would make sense to implement that first with the knowlegde that the functionality should be usable for the LSP, then we could reuse that functionality here. Until then, the distinction between public/private should be fine, but it can get tricky even then because you should be able to access private fields if you are within the scope of the declaration. We will also have fileprivate and internal (public for the module), so these further complicate things but for now you can assume they don't exist.

See hylo-lang/hylo-new#11