swr: bump minimum supported LLVM version to 4.0

swr: bump minimum supported LLVM version to 4.0

Since radv and radeonsi removed support for LLVM 3.9 the distcheck
target got broken because SWR distribution needed 3.9.x.

After checking with George Kyriazis, SWR is OK with moving to LLVM 4.0
and above, which will solve this problem.

Fixes: 3bf1e036e8a ("amd: remove support for LLVM 3.9")
Cc: George Kyriazis <george.kyriazis@intel.com>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: George Kyriazis <george.kyriazis@intel.com>