We're using a small custom frontend tool to generate indexstore data for `.swiftinterface` files in the SDKs. We do this by treating the `.swiftinterface` file as the input of an interface compilation, but this exits early because it treats it as a `SourceFile` instead of an external `LoadedFile`. This happens even if we call `setIsSystemModule(true)` unless we skip setting the SDK path, but that causes other problems. It seems harmless to check for `SourceFile`s as well, so that a tool processing an SDK interface as a direct input still gets the right state.
For unresolved member completion, we were preferring
the more general type, when we ought to be preferring
the more specific type. Additionally, for both
unresolved member and postfix completion we were
opening archetypes, which doesn't work as expected
since we don't compare requirements. Factor out
the logic that deals with merging base types for
lookup, and have it prefer either the subtype, or
the optional type in the case of optional promotion.
rdar://126168123
