swift234

Add the real ClangImporter as a dependency for sil-extract and sil-opt.

The standard library is about to start using some C/Objective-C
declarations internally, which means every tool needs to be import
C/Objective-C. (Don't worry, they're keeping it small.)

Apologies for your link times.

(The NullClangImporter is dead; I'll remove it next week.)

Swift SVN r14821