llvm: new version, 12.0.0 (#23033)
llvm: new version, 12.0.0 (#23033)

Co-authored-by: Robert Mijakovic <b72217041d23dc4c326868c532641758c08179ce@lxp.lu>
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack