[include-fixer] only deduplicate symbols after matching symbols with the undefined identifier.

Summary:
we should only deduplicate symbols after matching symbols with the
undefined identifier. This patch tries to fix the situation where matching
symbols are deleted when it has the same header as a non-matching symbol, which
can prevent finding the correct header even if there is a matched symbol.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21290

llvm-svn: 272576