PublicAPI.Unshipped.txt API files should be moved to PublicAPI.Shipped.txt

[gafter]

Now that we're past the C# 8 preview period and have shipped some new APIs, we should move the "Unshipped" API file to the "Shipped" file so we have a record of what APIs we shipped.

/cc @jcouv @agocke

[jcouv]

I think this should already part of @dotnet/roslyn-infrastructure plans (we don't have to do this for the compiler file specially). Let's confirm

[jasonmalinowski]

Hmm, at this point we'd be doing this for 16.2 GA since that's now shipped. Do you have anything else in 16.3 though that needs to be tracked now since you're "shipping earlier" from the perspective of a lockdown?

[333fred]

This was fixed in #38594.