llvm-cov: Rename MappingRegion to coverage::CountedRegion (NFC)

This name was too similar to CoverageMappingRegion, and the type
really belongs in the coverage library anyway.
llvm-cov: Combine two types that were nearly identical (NFC)

llvm-cov had a SourceRange type that was nearly identical to a
CountedRegion except that it shaved off a couple of fields. There
aren't likely to be enough of these for the minor memory savings to be
worth the extra complexity here.