llvm: don't build CppBackend for flang versions of llvm (#11841)
llvm: don't build CppBackend for flang versions of llvm (#11841)

The version of LLVM used by flang is new enough that CppBackend doesn't exist.  Unfortunately, `flang-xxxxxxxx` is seen as < `3.9.0` by the version check.  

* add a special case for `flang` versions.
iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack