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



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188404 91177308-0d34-0410-b5e6-96231b3b80d8