[RFC] Show output during clang report generation.

The last clang report build failed; looks like something doesn't work with current `neovim/neovim`, or maybe it just takes too much time and Travis times out. In either case, this should give us a clue of what's wrong. It basically prints *and* writes the output of scan-build to a file.

Build is at https://travis-ci.org/fwalch/bot-ci/jobs/40486113