It seems that when building from the LLVM master you can already enable the new driver by adding -DFLANG_BUILD_NEW_DRIVER=ON to the cmake incantation, see https://github.com/llvm/llvm-project/tree/master/flang#build-the-new-flang-driver
When building you may have to use --parallel N to reduce the number of parallel build steps because some targets require >=8GB of memory while linking.
And if I remember correctly Gary mentioning in his talk at FortranCon that the current state should be enough to start developing tools like flang-format, or some static analysis (the flang-doc build target is supposed to generate FIR documentation).