Incorrect nullability analysis for extension method with inference

In the example below, the only argument for the extension method is `oNull` which may be null.
So `oNull.Id` has signature `object? ()`.

The conversion to an explicit type works properly (warn on nullability issue with return type).
But in the function type scenario, it looks like `var x` isn't analyzed with that inferred type.

Also, the semantic model (`GetSymbolInfo`) has incorrect nullability for both member accesses.

This issue also impacts the semantic model for deconstructions.

This issue also impacts new extension members.

```c#
#nullable enable

object? oNull = null;
var x = oNull.Id;
x().ToString(); // Missing nullabiliy warning, indicating that the type inferred for `var` is incorrect

System.Func y = oNull.Id; // correct warning on return type of delegate type

static class E
{
public static T Id(this T t) => t;
}
```