swift234

[ClangImporter] Resolve forward class declarations to definitions in Swift.

When we see an unresolved @class in a Clang module, we check the class name
against the classes in the Swift module with the same name.

This unfortunately necessitates putting a reference to the active type checker
into the ClangImporter, because the class in the current Swift module hasn't
been type-checked yet. This is now being used everywhere we do a lookup.

<rdar://problem/16295994>

Swift SVN r15355