Produce coverage files that include info on non compiled functions

[davidanthoff]

The coverage info that is currently generated in practice needs to be post-processed with something like https://github.com/JuliaCI/CoverageTools.jl/blob/d8a703d61f969d372cc4bfe72538520f20c90b35/src/CoverageTools.jl#L152 to actually report what users would expect. It would be nice if that extra step could be unnecessary and if the coverage files that Julia generates just include 0s for lines that are in the source code but aren't compiled. Now that Julia can directly generate lcov files it would get us to a point where no post-processing of coverage info would be needed.