Most of the work was done in
https://abf.io/import/llvm/pull_requests/2
I have reworked clang-mandriva.patch to keeps it (as clang-rosa.patch).
It seems, it did not actually detect ROSA in the past (it checked that
/etc/lsb-release has DISTRIB_ID=Mandriva<...> while we have "Rosa<...>"
there). However, it could be useful to align the default Clang flags
with Ubuntu, Fedora and such.