cpp11-migrate: Transform now responsible for applying replacements

To make it possible for replacements made to headers as part of transforming
one translation unit to not be visible to the transform of other translation
units, Transform now handles replacement application as part of its
end-of-source handling. Several things were simplified as a result:

- The duplicated code in every transform for applying replacements is now gone
  and replaced with one location in Transform.
- RefactoringTool is no longer used since Transform houses the Replacements
  structure.
- RewriterContainer is now a private implementation detail of Transform (also
  renamed to RewriterManager since its behaviour is slightly different now with
  respect to lifetime of objects).
- There's now no distinction between input and output file state.

Misc notes:

- Interface changes reflected in unit tests.
- Replacements for files other than the main file are assumed to be for headers
  and stored as such.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@184194 91177308-0d34-0410-b5e6-96231b3b80d8
cpp11-migrate: Transforms honour header modification flag

Transforms will now make changes to headers if header modifications have been
enabled.

FIXME: Only UseNullptr contains a cursory header modification test. Other
transforms should have them too.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@184197 91177308-0d34-0410-b5e6-96231b3b80d8