swift234

[Driver] Handle outstanding jobs by reading the build record.

r23968 wrote out a record of which source files were included in a build,
and whether they were succesfully compiled or not...and if not, whether
they were out of date because of a cascading or non-cascading dependency.
This commit uses that information to decide what files might need to be
rebuilt even if a particular input doesn't change and doesn't appear to
have any changed dependencies. The two interesting cases are:

- A file was going to be built last time, but the build was halted
  because of an error. Build it this time.
- One of the files was removed and thus we've lost a source of dependency
  information; rebuild everything!

rdar://problem/19270980

Swift SVN r24018