Running apt-get update fails after installing some old Clang versions

[fbriere]

After installing one of the oldest Clang versions available (pre-5.0 on bionic, pre-9 on focal), any subsequent apt-get update will fail:

E: The repository 'http://apt.llvm.org/bionic llvm-toolchain-bionic-3.9 Release' does not have a Release file.
Error: Process completed with exit code 100.

It appears that these versions are not present on apt.llvm.org, even though they are in universe.

(setup-clang itself ignores this failure and carries on, though I'm not sure if this was intended on your part.)

It would be nice if the repo URL was checked first and omitted if missing. Or, failing that, a mention in the README advising running any apt-get update before setup-clang would help prevent some head-scratching. Thanks!