swift234

[PrintAsObjC] Guard @import lines with __has_feature(modules).

Also, use "#include <objc/NSObject.h>" instead of "@import ObjectiveC;"
to get access to BOOL, SEL, NSObject, and NSString.

This allows generated headers to be used in Objective-C++ contexts, where
modules don't yet work. The dependencies will unfortunately need to be
imported separately (because there's not a direct mapping from module name
back to header file), but that's still better than just being incompatible.

<rdar://problem/16796627>

Swift SVN r17272