Suggest type annotations for partial types spanning multiple scopes

[JukkaL]

We don't support inferring partial types from multiple scopes in fine-grained incremental mode (#4553), but this works in other modes. Migrating to fine-grained incremental mode will often require adding some annotations. It would make sense to automatically generate suggestions for the type annotations (at least in many common cases) to simplify the migration.

Original issue with discussion: #4492, especially #4492 (comment)