Completion list doesn't include local variable that is later shadowed in the same scope

Introduced in #880 .

[Playground link ](https://microsoft.github.io/qsharp/?code=H4sIAAAAAAAACo2OuwrCQBBF%2B3zFYJVUFnYJgiAWFmoMdiGQZR11ITsTdmeLIPl3Y%2FCBYuGthsucwyVl0bdKI2y6fVAkweaOz05ZuEYRDFmsSFyXsyGJk7HhFp0SwwQbZShOIIUCfWikrJ7QPdMpaHYOtaTDYdsGR6YxXuDI6IFYwJBuwhGhPjHXL3R4haGAOcyyD%2BF6u9wVxWp5%2BFv5wzm5mEn2386Hy3%2Fte8MOJTiCssrGqo%2F66AZaE30ZUwEAAA%3D%3D)

```qsharp
namespace MyQuantumProgram {
@EntryPoint()
operation Main() : Result[] {

// correct: completion list does not include `foo`
let foo = 3;
// INCORRECT: completion list does not include `foo`
let foo = "hi";
// correct: completion list includes `foo`

return [];
}
}
```

When the cursor is in between the two declarations, we should show `foo` in the list, with type `Int`. Instead, today, because we have set the "valid at" offset based on the last declaration in the scope, we don't show `foo` anywhere above that last declaration.

Start with the linked Playground example and trace the completion logic that sets the valid-at offset for declarations in one scope. The fix is done when completion between the two `foo` declarations includes `foo` with type `Int`, while completion after the second declaration continues to reflect the later shadowing.

Written by the indexing model from the issue text.