Add support to the Clang driver for forwarding all of gfortran's flags
to GCC when asked to compile a fortran input.

This fixes a regression with essentially every Fortran compile since we
started rejecting unknown flags. Also moves a mis-classified gfortran
flag into the nicely documented set.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192867 91177308-0d34-0410-b5e6-96231b3b80d8