[include-fixer] Rank symbols based on the number of occurrences we found while merging.

This sorts based on the popularity of the header, not the symbol. If
there are mutliple matching symbols in one header we take the maximum
popularity for that header and deduplicate. If we know nothing we sort
lexicographically based on the header path.

Differential Revision: http://reviews.llvm.org/D20814

llvm-svn: 271283