swift234

[Driver] Add basic support for private dependencies.

- Add flags to dependency entries in DependencyGraph.
- Don't traverse past private dependencies in markTransitive.
- Only mark dependent jobs after a build if the build was triggered
  (a) explicitly (because the file is out of date), or
  (b) because of a non-private dependency.

This still isn't fully correct because of new non-private dependencies
discovered /after/ building an individual file, but it's on the way there.
Solving that problem will require tracking which dependencies have already
been marked dirty (next commit).

Swift SVN r23852