CI: update llvm apt sources to correct ubuntu version

In the pipelines.yml file we request Ubuntu 18.04 Bionic Beaver, but in
the script we were still using the Xenial apt.llvm.org sources.
9daa7e1e19Andrew Kelley committed on 12/20/2019, 6:40:07 PMparent4d54e9a
1 file changedLine totals unavailable