[Coverage] Make sorting criteria for CounterMappingRegions local.

Move the comparison function into the only place there it is used,
i.e. the call to std::stable_sort in CoverageMappingWriter::write().

Add sorting by region kinds as it is required to ensure stable order
in our tests and to simplify D23987.

Differential Revision: https://reviews.llvm.org/D24034

llvm-svn: 280198