CMakeLists.txt: require LLVM 9.0 or later
CMakeLists.txt: require LLVM 9.0 or later

Signed-off-by: Matt Madison <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@madison.systems>

madisongh/blissc,madisongh/blissc,madisongh/blissc