Classic Flang (GitHub - flang-compiler/flang: Flang is a Fortran language front-end designed for integration with LLVM.) is a separate compiler and not shipped along with llvm.
flang-new is the driver of the Fortran frontend shipped along with llvm. flang is a shell script that can parse, perform semantics, and unparse and call an external compiler to do the codegen. flang basically calls flang-new internally. When codegen becomes available then the flang script will be removed/renamed and flang-new will be renamed as flang.
I don’t know why the iso_c_binding usage is not working. I expected it to work. Maybe you can file a bug (Bug List).