Fixes https://github.com/dotnet/roslyn/issues/37252

Fixes #35933

Currently, the APIs for nullable reference types take a model that
instead of the top level nullability of a named type being on the
INamedTypeSymbol, it's carried along as a separate piece of information.
This causes a lot of churn for code that is used to passing along
ITypeSymbols today; one way to ease this is simply create a set of
wrapper types which carry it along for us but still implement
ITypeSymbol.