Allow closures to read a protected base field

[T-Gro]

Fixes #5302

// C#: class BaseClass { protected string ProtectedField; }
type DerivedClass() =
    inherit BaseClass()
    member x.Run() = let f () = x.ProtectedField in f ()   // was FS1097

Object expressions stay the boundary — the override body is a separate type, not nested under DerivedClass:

{ new BaseClass() with
    override this.ToString() = (lazy this.ProtectedField).Value }   // still FS1097

--langversion:preview

[github-actions]

⚠️ Release notes required, but author opted out

Warning

Author opted out of release notes, check is disabled for this pull request.
cc @dotnet/fsharp-team-msft