Add a SourceContext.InClass overload accepting a Typ
Add a SourceContext.InClass overload accepting a Typ

The approach of creating a Typ from the namespace and identifier fails if we're creating a nested type.
In most cases it's fine, and it probably *could* be fixed, but for the moment, the overload is the simplest approach.

googleapis/gapic-generator-csharp,googleapis/gapic-generator-csharp