Implement hierarchy index in suggestion database (#3992)

[Task link](https://www.pivotaltracker.com/story/show/184012397).

This PR adds a hierarchy index to our suggestion database. The index will be used in the future when building documentation pages. E.g., documentation of the Module includes all the Types defined in this module; documentation of the Type includes documentation for every Method and Constructor defined for this type.

We could call this index "self-type index", but the Type-Module relation is not using self-type.

No visual changes to the IDE were made.

# Important Notes
We rely on the order of updates from the engine. In particular, the following scenario would lead to inconsistency in the DB:
1. Engine sends an update, changing the parent module of some type to "NonExistentModule"
2. Engine sends an update, adding a new entry "NonExistentModule"

I assume that the engine would never send the update with a non-existent qualified name used.