[include-fixer] Refactor include fixer to be usable as a plugin

- Refactor the external sema source into a visible class
- Add support for emitting FixIts
- Wrap up include fixer as a plugin as I did with clang-tidy

Test case will follow as soon as I wire this up in libclang.

Differential Revision: https://reviews.llvm.org/D26752

llvm-svn: 287228