In the 6.0.1 -> 7.0.0 LLVM migration of
f27639babf, archive hashes were added for
the LLVM 7.0.0 binaries not only for Ubuntu 16.04, but also for Ubuntu
14.04. The DownloadAndExtractClang.cmake functionality would never allow
making use of the latter, however; always using the Ubuntu 16.04 LLVM
7.0.0 binary in case of OS Linux.
This commit generalizes the archive name derivation for when the current
platform is Ubuntu, while persisting using Ubuntu 16.04 as default
archive in case CMAKE_SYSTEM_NAME is Linux (even if the actual dist is
not Ubuntu).
This change will also facilitate keeping backwards compatibility with
Ubuntu 16.04 (possibly 14.04) for the upcoming LLVM 8.0.0 migration,
where an archive for Bionic (18.04) is expected to be included (which may
be chosen as the default).
