[include-fixer] Implement adding missing namespace qualifiers in vim integration.

Summary:
The patch extends include-fixer's "-output-headers", and "-insert-headers"
command line options to make it dump more information (e.g. QualifiedSymbol),
so that vim-integration can add missing qualifiers.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D22299

llvm-svn: 275279