meson: Force the use of config-tool for llvm

meson git now has a cmake find method for llvm, but it lacks a couple of
features that we use from the config tool version. Until that reaches
parity we need to use the config-tool version.

CC: 19.0 19.1 <<mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 0d594594)