cpp11-migrate: Remove mention of 'headers' from serialization code

* HeaderChangeDocument -> MigratorDocument
* HeaderFileName -> TargetFile
* SourceFileName -> MainSourceFile
* Removed TransformID
* Comments updated, at least with respect to serialization
* Unit tests updated.

Differential Revision: http://llvm-reviews.chandlerc.com/D1403

llvm-svn: 188404