[include-fixer] Add missing namespace qualifiers after inserting a missing header.

Summary:
This is an initial version of fixing namespace issues by adding missing
namespace qualifiers to an unidentified symbol.

This version only fixes the first discovered unidentified symbol.
In the long run, include-fixer should fix all unidentified symbols
with a same name at one run.

Currently, it works on command-line tool. The vim integration is not
implemented yet.

Reviewers: klimek, bkramer, djasper

Subscribers: bkramer, ioeric, cfe-commits

Differential Revision: http://reviews.llvm.org/D21603

llvm-svn: 274832