Records: Should produce an error when member we pick as positional member is hidden

[jcouv]

In the following example, we use Base.I as the positional member (ie. it is used as part of C.Deconstruct synthesized method). But it is hidden by the method C.I.
From discussion in LDM 4/14/2021, we decided to make this an error and take this as a breaking change for 16.10.

public record Base
{
    public int I { get; set; } = 42;
    public Base(int ignored) { }
}
public record C(int I) : Base(I)
{
    public void I() { } // hiding
}