[llvm-cov] Factor out logic to iterate over line coverage stats (NFC)

There were two copies of the logic needed to construct a line stats
object for each line in a range: this patch brings it down to one. In
the future, this will make it easier for IDE clients to display coverage
in-line in source editors. To do that, we just need to move the new
LineCoverageIterator class to libCoverage.

llvm-svn: 315789