LLVM C++ standard library
copied from cf-staging / libcxx-devel
conda install conda-forge::libcxx-develconda install conda-forge/label/broken::libcxx-develconda install conda-forge/label/libcxx_debug::libcxx-develconda install conda-forge/label/libcxx_dev::libcxx-develconda install conda-forge/label/libcxx_macos_lt_12::libcxx-develconda install conda-forge/label/llvm_dev::libcxx-develconda install conda-forge/label/llvm_rc::libcxx-devel