Proposal: add nullable reference type support for LINQ query syntax

[roji]

No warnings are reported on dereferencing of NRT variables within query syntax expressions:

Student?[] students = new[]
{
    new Student { Name = "foo", DepartmentId = 1 },
    new Student { Name = "bla", DepartmentId = 2 },
    null
};

Department?[] departments = new[]
{
    new Department { Id = 1, Name = "bar" },
    null
};

// The produces warnings on s and d (and fails at runtime), since students and departments are nullable
_ = students.Join(departments, s => s.DepartmentId, d => d.Id, (s, d) => s.Name.Length + d.Name.Length).ToArray();

// This produces no warnings, although it does the same thing
var query =
    from s in students
    join d in departments on s.DepartmentId equals d.Id
    select s.Name.Length + d.Name.Length;

_ = query.ToList();

class Student
{
    public required string Name { get; set; }
    public int DepartmentId {get; set;}
}

class Department
{
    public int Id { get; set; }
    public required string Name { get; set; }
}