cpp11-migrate: Transform now responsible for file content overriding

To better support per-translation unit replacements, any real work is being
moved out of ActionFactory and into Transform. In this revision, that means
file override application.

For simplification, Transform no longer inherits from SourceFileCallbacks.
TransformTest required updating as a result.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@184098 91177308-0d34-0410-b5e6-96231b3b80d8