typealias handling could be better

[stephen-hawley]

I've added the notion of type aliases into the the type database.
You can associate with a module a list of type aliases defined on types in the module. The only example of that (so far) is Foundation.NSRect which maps onto CoreGraphics.CGRect.

What needs to be done is:
go through the public Swift API and find all the type aliases
add them to the type database (right now, swiftCore.xml)

Optional:
Consider if we want more type database files than swiftCore.xml.
Organizing the type databases is less of an issue since every entry in type databases has a module name so they'll end up in the right place no matter what, but it might be nice to do the right thing and organize the elements where they belong.