swift234

[Driver] Schedule dirty-because-dependent jobs more eagerly.

Previously, the driver waited for the first set of known-dirty jobs to
finish before doing /any/ dependency analysis. This was correct, but
could take a lot longer (consider waiting for one touched file to compile
and then finding out that it affects three others, when all four could
have been built in parallel). This only affects the incremental build.

Swift SVN r23967