[tooling] FixItHint Tooling refactoring

Summary:
This is the refactoring to lift some FixItHint into tooling.
used by: http://reviews.llvm.org/D19807

Reviewers: klimek, alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19941

llvm-svn: 269188