Immutability analyzer doesn't see destructuring assignments

[j3parker]

[Immutable]
public sealed class Whatever {
  private readonly object x;

  public Whatever() {
    x = new object(); // ok
    (x) = new ValueTuple<object>(new List<string>()); // needs to be caught
  }
}

while we're here, add a test case for the left hand side being parenthesized but not destructuring: