swift234

[Driver] Eliminate the common base class of Command and JobList.

...and rename Command to Job (previously the name of the base class).

We never generated job lists directly contained in other job lists, so
let's not even worry about this case. We may some day need to break Job
out into separate subclasses (Clang has Command and FallbackCommand in
addition to JobList), but we should be able to keep the list separate.

No intended functionality change.

Swift SVN r23144