Implement tooling::Replacements as a class.

Summary:
- Implement clang::tooling::Replacements as a class to provide interfaces to
  control how replacements for a single file are combined and provide guarantee
  on the order of replacements being applied.
- tooling::Replacements only contains replacements for the same file now.
  Use std::map<std::string, tooling::Replacements> to represent multi-file
  replacements.
- Error handling for the interface change will be improved in followup patches.

Reviewers: djasper, klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D21748

llvm-svn: 277335