[flang] Add cmake option to link with FIR and LLVM

The cmake option -DLINK_WITH_FIR=ON causes the f18 driver to be linked
against FIR and LLVM. When it is set to OFF (the default), the call into
FIR is #ifdef'd out.

The source in lib/FIR is always built; this option only affects linking.

Original-commit: flang-compiler/f18@69569edd4c6abbb7a66c0d8f3595858693a06996
Reviewed-on: https://github.com/flang-compiler/f18/pull/329