[Driver] Define LinkOption and fix forwarded options to GCC for linking

Many driver options are neither 'DriverOption' nor 'LinkerInput'. When gcc is
used for linking, these options get forwarded even if they don't have anything
to do with linking. Among these options, clang-specific ones can cause gcc to
error.

Just use 'OPT_Link_Group' and a new flag 'LinkOption' for options which already
have a group.

gfortran support apparently bit rots (which does not seem to make much sense). XFAIL the test.