Handle nullable postcondition for non-static local function inside field initializer

[RikkiGibson]

Closes #76528

[jjonescz]

Consider testing also when the lambdas are marked static.

[RikkiGibson]

@dotnet/roslyn-compiler for second review

[AlekseyTs]

// Functions inside a field initializer are effectively static.

Is this accurate? For example, it looks like in the following scenario both the lambda and the local function are implemented by an instance method.

#nullable enable

using System;
using System.Diagnostics.CodeAnalysis;

public class C(string p)
{
    public static string? field;

    public Action field2 = () =>
    {
        init();
        Console.WriteLine(field.Length);

        [MemberNotNull(nameof(field))]
        void init() => field ??= p;
    };
    
    string M() => p;
}

[RikkiGibson]

You're right, when marked static the functions cannot use p. I'll adjust the comment and add the test.

[AlekseyTs]

It looks like the comment got removed, but it feels like we still need some rational to explain why returning 0 is probably not 100% accurate, but OK thing to do.

[AlekseyTs]

Done with review pass (commit 2)