swift234

[Driver] Only run compile jobs if needed.

This teaches the driver's Compilation to not run jobs where the base input
is older than the main output (r23221) when we're tracking dependencies.
After a compile command finishes, anything that depended on the file that
just got compiled will get scheduled.

This has the nice side effect of trying to rebuild changed files first.

The tests here aren't really testing the dependency graph yet, because the
files don't include any dependencies. I'll be adding several more test
scenarios in the next few commits.

Part of rdar://problem/15353101

Swift SVN r23273