swift234

[ClangImporter] Don't force-load Foundation under ImportForwardDeclarations.

When we print the ObjectiveC module, we really shouldn't see extensions that
come from Foundation. Admittedly, we don't see them correctly in Foundation
either, but that's a separate issue.

There is one slightly unsafe hack here: we still import NSString as String,
by name alone, even when Foundation isn't present. This is because NSString
shows up in the ObjectiveC module, but we want people to be using String
when interacting with NSObject in real life.

Swift SVN r21857