swift234

[driver] Reworked Driver::buildJobsForAction() so that it doesn’t create the same Job multiple times.

Since a Job may be reached through multiple paths in the dependency graph, we
need to include support for caching Jobs. (Currently, the cache maps
Action/ToolChain pairs to Jobs.)

As a result of these changes, Tool::constructJob() and
Driver::buildJobsForAction() return a plain Job *, instead of a
std::unique_ptr<Job>, since the Jobs returned by buildJobsForAction() no longer
need to be unique.

Swift SVN r12443