swift234

[Driver] A .swift file should be rebuilt if its mtime has changed.

...not if it's newer than its output .o file. This handles cases where the
object file is generated too quickly (rdar://problem/19404140) or when you
revert to a previous version of the file, mtime intact (rdar://problem/19720146).

There's a lot of test churn here; the only real new test is the backwards
mtime update in one-way.swift.

Swift SVN r29584