LineCoverageIterator is not providing access to a mutable object. Fix it
to iterate over `const LineCoverageStats` so that `operator->()`
compiles again after 6b9b86db9d.
LineCoverageIterator is not providing access to a mutable object. Fix it
to iterate over `const LineCoverageStats` so that `operator->()`
compiles again after 6b9b86db9d.