Add Accessibility modifier removes "ref" from a ref struct

[VSadov]

Invoke Refactor/Add Accessibility modifiers on

readonly unsafe ref struct S1 { }

The dialog offers adding "private", but also offers to remove "ref"

[OmarTawfik]

Root cause: classes in src/Workspaces/CSharp/Portable/CodeGeneration don't use compiler helpers to generate refness of signatures, but instead do independent checks. Need to look deeper into that.