[include-fixer] Support processing multiple files in one run.

Summary:
Previously, if we pass multiple files or a file pattern (e.g. /path/to/*.cc) to
include-fixer, include-fixer will apply all replacements to the first argument,
which probably causes crashes.

With this patch, include-fixer can process multiple files now.

Vim and Emacs integration are tested manually.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23266

llvm-svn: 278102