[PATCH] modularize - header dependencies - version 2

[Doug Gregor]

  The implementation looks fine.

  Regarding Sean's question: applying modularize to a set of system headers can encounter a huge number of problems, not all of which are easy to fix. Adding a feature like this to modularize can help someone work around issues in the headers (which can also be worked around in the module map by listing headers in the appropriate order) to get better results from modularize with fewer widespread changes. For that reason, I think this is a useful feature to have. If we could fix all of the world's system headers, we would :(

http://llvm-reviews.chandlerc.com/D1474