swift234

[Driver] Add several tests for the DependencyGraph.

malformed: Compiling generates a broken swiftdeps file -> recompile everything.
mutual: Two files depend on each other -> compiling either forces a recompile.
one-way: One file depends on another
  -> the latter forces the former to be recompiled.
one-way-depends-before: One file starts off depending on another
  -> the latter forces the former to be recompiled, but only the first time.
one-way-depends-after: One file newly depends on another
  -> the latter forces the former to be recompiled, but only once the former
     has been touched.
one-way-provides-before: One file starts off providing a dependency for another
  -> the former forces the latter to be recompiled, but only the first time.
one-way-provides-after: One file newly provides a dependency for another
  -> the former forces the latter to be recompiled.
simple-half-dirty: Two independent files -> recompile only the dirty one.

Swift SVN r23275