Record constructor arguments should propagate nullability to properties

[RikkiGibson]

Version Used:
f36e10a

Steps to Reproduce:

#nullable enable
data class B(string? X, string? Y)
{
    static void M1(bool flag)
    {
        B b = new B(""hello"", null);
        if (flag)
        {
            b.X.ToString(); // shouldn't warn
            b.Y.ToString(); // 1
        }

        b = b with { Y = ""world"" };
        b.X.ToString(); // shouldn't warn
        b.Y.ToString();
    }
}

Expected Behavior:
Warning only on line // 1

Actual Behavior:
Warnings on line // 1 as well as // shouldn't warn lines

[RikkiGibson]

The main blocker on this is that we don't have a decent way to tell if a constructor is the record primary constructor. We can in source, but not so easily in metadata. We could go forward by just assuming that every record constructor parameter assigns to an identically-named property, but this seems dubious.

[RikkiGibson]

Solution to this issue should also address deconstruction as described in #49741