swift234

[Driver] Add a new DependencyGraph class.

...and some basic unit tests for it.

The purpose of this class is to track dependencies between opaque nodes.
The dependency edges are (kind, string) pairs, where the "kind"
distinguishes different kinds of dependencies (currently "top-level names"
and "types that we do lookup on"). The step is to make use of it in
running compile commands.

The YAML-based file format is only for bring-up and testing purposes.
I intend to switch it to a bitcode-based format in the long run.

Part of rdar://problem/15353101

Swift SVN r23223