PS-7704: Travis CI: fix installation of clang-9 on Ubuntu Bionic
PS-7704: Travis CI: fix installation of clang-9 on Ubuntu Bionic

https://jira.percona.com/browse/PS-7704

Do not try to install `clang++` which matches a regex.
This is a copy of similar fix for PS from
https://github.com/percona/percona-server/commit/952e65845d1813b221ca9713a2c0b2ba805b67e7.

percona/PerconaFT,percona/PerconaFT,percona/PerconaFT,percona/PerconaFT