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)