[find-all-symbols] Save relative file path for each symbol.

Summary:
The HeaderSearch::suggestPathToFileForDiagnostics used in include-fixer
doesn't work well with absolute file path, it assumes a relative file
path based on header searching path.

So saving relative file path to make include-fixer get the most
appropriate header path.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20158

llvm-svn: 269173