There isn't much that hasn't been open sourced in the past few years: The runtime, standard library, C#/F#/VB compilers, ASP.NET, Entity Framework, Xamarin, PowerShell, Winforms, WPF are all open source or (in the case of WPF) being open sourced.
Roslyn is the compiler. There's not much of a debugger in it, is there? I mean, I guess the debugger will use Roslyn to parse watch expressions and evaluate things in the Immediate Window, but beyond that I think debugging is handled by something else.