swift234

[Driver] Use -incremental as the top-level option for dependency tracking.

This was being staged as -emit-reference-dependencies, but it's affecting
a lot more than that. Eventually for command line builds this should also
preserve intermediate build outputs (like .o and .swiftmodule) for use in
later builds, rather than putting them in $TMPDIR and deleting them after.

This option is still hidden.

Swift SVN r23295