APT whitelist request for clang-6.0 in trusty

[Zialus]

Same as #4270 but with correct title.

[travisbot]

This is an automated comment.

Ran tests, but could not found source package. Either the source package for clang-6.0 does not exist, or the package needs an APT source. If you wish to add an APT source, please follow the directions on https://github.com/travis-ci/apt-source-whitelist#source-approval-process. Build results: https://travis-ci.org/travis-ci/apt-whitelist-checker/builds/375445019.

[travisbot]

This is an automated comment.

Ran tests, but could not found source package. Either the source package for clang-6.0 does not exist, or the package needs an APT source. If you wish to add an APT source, please follow the directions on https://github.com/travis-ci/apt-source-whitelist#source-approval-process. Build results: https://travis-ci.org/travis-ci/apt-whitelist-checker/builds/440498131.