Support report from child pipeline for MR coverage report

Implement support for report for child pipeline for MR coverage report, as part of #362876 (closed).

Proposal

In http://gitlab.com/gitlab-org/gitlab/blob/dde3a70e1d9c21f0f5698d16f3f0f239b0836a4a/app/controllers/projects/merge_requests_controller.rb#L132-L132, we expose coverage path to the frontend using merge_request.has_coverage_reports?, which in turn checks if the head pipeline has code coverage pipeline artifacts. http://gitlab.com/gitlab-org/gitlab/blob/dde3a70e1d9c21f0f5698d16f3f0f239b0836a4a/app/models/ci/pipeline.rb#L1072-L1074

This in turn is used to return the actual coverage report in http://gitlab.com/gitlab-org/gitlab/blob/dde3a70e1d9c21f0f5698d16f3f0f239b0836a4a/app/services/ci/generate_coverage_reports_service.rb#L15-L15.

So to support the reports from child pipeline, we need to

1. Introduce Pipeline#has_reports_in_self_and_descendants? and Pipeline#latest_report_builds_in_self_and_descendants which accounts for reports in the descendant pipelines.
2. Generate coverage report pipeline artifact from pipeline and descendants in http://gitlab.com/gitlab-org/gitlab/blob/dde3a70e1d9c21f0f5698d16f3f0f239b0836a4a/app/services/ci/pipeline_artifacts/coverage_report_service.rb#L4-L4. using the inclusive methods added above.