cpp11-migrate: Refactor for driver model of operation

Massive simplification of how replacements and file overrides are handled by
the migrator:
* Sources and headers are all treated the same.
* All replacements for a given translation unit are stored in the same
  TranslationUnitReplacements structure.
* Change tracking is updated only from main file; no need for
  propagating "is tracking" flag around.
* Transform base class no longer responsible for applying replacements.
  They are simply stored and main() looks after deduplication and
  application.
* Renamed -yaml-only to -serialize-replacements. Same restrictions apply:
  Can only request one transform. New restriction: formatting cannot also
  be turned on since it's basically a transform.
* If -serialize-replacements is requested, changes to files will not be
  applied on disk.
* Changed behaviour of function generating names for serialized
  replacements: Only the main source file goes into the name of the file
  since a file may contain changes for multiple different files.
* Updated HeaderReplacements LIT test for new serialization behaviour.
  * Replaced old test that ensures replacements are not serialized if
    -serialize-replacements is not provided. New version ensures changes
    are made directly to all files in the translation unit.
* Updated unit tests.
  * Due to major simplification of structures in FileOverrides.h, the
    FileOverridesTest is quite a bit simpler now.

Differential Revision: http://llvm-reviews.chandlerc.com/D1545