ac/llvm: fix build with LLVM 17

ac/llvm: fix build with LLVM 17

This builds with LLVM 12 -> 17 and a running a simple app seems to work.

I couldn't test LLVM 11 because meson fails with:

    Looking for a fallback subproject for the dependency llvm (modules:
    bitwriter, engine, mcdisassembler, mcjit, core, executionengine,
    scalaropts, transformutils, instcombine, amdgpu, bitreader, ipo, native)

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8297
Cc: mesa-stable
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22021>