Typically, users jump to type-specific interface from a member of that type, for
instance, a.getSomething(). To generate the interface, we need to report the USR
of the container type of "getSomething()", which is the USR for the type of a,
when cursor info is requested for this function call.
When users try to print the interface of a specific type (most often through cursor
infor query of SourceKit), we should simplify the original decls by replacing
archetypes with instantiated types, hiding extension details, and omitting
unfulfilled extension requirements. So the users can get the straight-to-the-point
"type interface". This commit builds the testing infrastructure for this feature,
and implements the first trick that wraps extension contents into the interface body.
This commit also moves some generic testing support from SourceKit to Swift.
Swift SVN r32630
