meson: Check for actual LLVM required versions

meson: Check for actual LLVM required versions

Currently we always check for 3.9.0, which is pretty safe since
everything except radv work with >= 3.9 and 3.9 is pretty old at this
point. However, radv actually requires 4.0, and there is a patch for
radeonsi to do the same.

Fixes: 673dda833076 ("meson: build "radv" vulkan driver for radeon hardware")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>