Nullable analysis should not give conditional access placeholders their own slot

[RikkiGibson]

Follow up to #75554

[Fact]
public void NullableAnalysis_02()
{
    // The expected warning is missing here.
    // Problem: the conditional receiver and its field are getting their own slots.
    // But, when the assignment '.F = null' is processed, we do look up the slot for 'c' thru 'NullableWalker._lastConditionalAccessSlot'.
    // Thus the state for 'c.F' gets updated to maybe-null, but the state for '<placeholder>.F' remains not-null.
    // When we get a slot for RHS of next 'c?.F' expression, we get the slot for '<placeholder>.F', and see the .F as having not-null state.
    // We may want to solve this by ensuring we don't create a slot for the placeholder, and that getting a slot for the placeholder always gives the slot for the original receiver instead.
    var source = """
        #nullable enable

        class C
        {
            string? F;

            static void M1()
            {
                var c = new C { F = "a" };
                c.F.ToString();
                c?.F = null;
                c?.F.ToString(); // 1
            }
        }
        """;

    var comp = CreateCompilation(source);
    comp.VerifyEmitDiagnostics();
}