[include-fixer] Create a mode in vim integration to show multiple potential headers.

Summary:
Some changes in the patch:

* Add two commandline flags in clang-include-fixer.
* Introduce a IncludeFixerContext for the queried symbol.
* Pull out CreateReplacementsForHeader.

Reviewers: bkramer

Subscribers: klimek, cfe-commits, ioeric

Differential Revision: http://reviews.llvm.org/D20621

llvm-svn: 271258