chore(deps): clover: raise the minimum LLVM version to 11.0.0

chore(deps): clover: raise the minimum LLVM version to 11.0.0

LLVM 11 was released in October 2020. If you want to build against
Mesa's Git version, that seems like enough time to upgrade to at least
LLVM 11 (Debian stable has this too).

It reduces the amount of #if gates we need and more will be incoming
again, given the Opaque Pointer transition.

Additionally radeonsi is already requiring LLVM 11. Therefore the
minimum will have been LLVM 11 for many builds anyway.

Note that clc is kept to LLVM 10 for the time being.

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16047>