Fix some issues reported by lintian

Fix some issues reported by lintian

• Bump debhelper from old 9 to 12. (package-uses-old-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)

This merge proposal was created automatically by the Janitor bot.

You can follow up to this merge proposal as you normally would.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/llvm-defaults/49aacfdf-6ef8-4a31-8a79-ab2979828e97.

These changes affect the binary packages; see the debdiff

You can also view the diffoscope diff (unfiltered).