Until now an unoffcial mirror was used, which may or may not
be maintained in the longer run, as the need for it has gone.
Also, the official mirror has version tags which allows us
to track upstream development in an automated manner.
${S} has changed as upstream decided to use a monorepo for all
of the llvm-related projects.
Further info:
https://www.phoronix.com/scan.php?page=news_item&px=LLVM-GitHub-Mono-Repo
https://llvm.org/docs/GettingStarted.html#checkout-llvm-from-git
Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
