[clang-tidy] Fix IncludeInserter/IncludeSorter bug.

If there weren't any includes in the file, or all of them had 'IncludeKind'
greater than the desired one, then 'CreateIncludeInsertion' didn't work.

http://reviews.llvm.org/D12017

Patch by Angel Garcia!

llvm-svn: 245042