swift234

[Driver] When a command finishes, don't try to reschedule ALL other commands.

Instead, if we can't schedule a command, record why it was blocked. When the
blocking command completes, we then try to reschedule everything that was
blocked on it.

This is also more robust for cross-job-list dependencies---things like the
link job depending on the merge-module job and both depending on compile jobs.

Swift SVN r23143