pattern_analysis: rework how we hide empty private fields

Consider this:
```rust
mod foo {
  pub struct Bar {
    pub a: bool,
    b: !,
  }
}

fn match_a_bar(bar: foo::Bar) -> bool {
  match bar {
    Bar { a, .. } => a,
  }
}
```

Because the field `b` is private, matches outside the module are not allowed to observe the fact that `Bar` is empty. In particular `match bar {}` is valid within the module `foo` but an error outside (assuming `exhaustive_patterns`).

We currently handle this by hiding the field `b` when it's both private and empty. This means that the pattern `Bar { a, .. }` is lowered to `Bar(a, _)` if we're inside of `foo` and to `Bar(a)` outside. This involves a bit of a dance to keep field indices straight. But most importantly this makes pattern lowering depend on the module.

In this PR, I instead do nothing special when lowering. Only during analysis do we track whether a place must be skipped.

r? `@compiler-errors`