Issue Description:

https://github.com/vapor/api-template @ af8148b447a16f22db819737ad97da9beef634f5
https://github.com/nodes-vapor/jwt-keychain @ 55dd5703e90c8c673ac62051aca312541a8c40f2

These two projects show examples of a regression in the Swift 5.2 compiler causing many Vapor 3 projects to not compile. To reproduce the bug, compile those commit hashes with Swift 5.2.

The regression is based around this code:

public protocol Model {
    associatedtype Database
}

Each database package introduces types like:

public protocol MySQLModel: Model
    where Self.Database == MySQLDatabase 
{ }

In 5.1 and earlier this allowed for declarations of models to look like:

final class Todo: SQLiteModel {
    ...
}

However, with 5.2, the `Database` typealias must be provided even though it is constrained to a specific type by the protocol being conformed to.

final class Todo: SQLiteModel {
    // Swift 5.2 requires this even though SQLiteModel constrains Database
    // to this exact type. 
    typealias Database = SQLiteDatabase
    ...
}