swift234

[Driver] Write out a record of which files were compiled in a build.

This is important because we might get part-way through the full
compilation, overwriting swiftdeps files as we go, and then encounter an
error. We don't want to lose information about any decls that have been
removed since the previous compile, so we propagate forward the information
we already have by saving it to a "build record" file.

More simply, this is necessary to track when a file is removed from a target.

The next commit will handle reading in this file at the start of a build.

Swift SVN r23968