swift234

Remove LoadedModule, and strip ModuleLoader down to just loading things.

Now that everything is done in terms of FileUnits, we don't need LoadedModule
anymore, and now that FileUnits just use virtual dispatch, we don't need to
indirect through ModuleLoader to distinguish them.

This doesn't quite simplify as much as it could, because the next change is
going to combine TranslationUnit and Module.

Swift SVN r10836