swift234

[Driver] Add an mtime test for single-file compilation jobs.

The Swift compiler is always fed the entire list of files in a module.
If it's told to track dependencies, though, it should look to see if it
actually needs to recompile all of its inputs. The first step in this is
to see which files are actually dirty, which it does by comparing the mtime
of each source file with the mtime of its output object file. If a source
file is not dirty, it only needs to be rebuilt if it depends on something
in a dirty file.

Nothing actually uses this information yet, but we can print it with
-driver-print-bindings!

Swift SVN r23221