Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/961596

Roslyn is responsible for applying SubType to Forms, Components and other types. In multi-targeted projects, the same file can have conflicting "sub types":

#if NET45
    class MyComponent : Form
#else
    class MyComponent : Component
#endif

This results Roslyn pushing "Form" when it encounters scans the first definition and "Component" when it encounters the second definition.
There is no guaranteed order to which one of these get scanned first, so this can result in the same time having a different sub type every time Roslyn does analysis. Sub Type affects the designer and icon that a type gets, so this results in a type that flickers between two different icons.

This was worse before dotnet#5880 as we'd see the sub type change and go back to Roslyn to add/remove the file, resulting in a constant churn. This longer occurs, but we still have the fighting sub type dance whenver the file content changes.

To fix this, Roslyn will now only scan the the primary context: dotnet/roslyn#44270.

This change plumbs through this property by finding the context that lives in the "active configuration" and passing this through to Roslyn.