CI coverage issues and trying out the llvm coverage tool

@clalancette Thank you for comment.

You should always look at the nightly jobs, i.e. nightly_linux_coverage [Jenkins]

Thank you, I do so.

you need to be logged into the Jenkins instance in order to access them. I’ve asked the internal infrastructure team to see if we can lift that restriction.

Can I create account or log into the Jenkins? I thgouth only OSRF or on-boaring member can do it.
Anyway, it looks better if we can lift that restriction.

Filter out the non-src directories from the listing.

We may use lcov --remove option.
I got the following output by this command in my environment.

lcov --remove cov_rclcpp_exclude -o cov_rclcpp_exclude2.info \
  '*/build-cov/*' '*/install-cov/*' '*/test/*'