[clang] [flang] Ignore -f[no-]realloc-lhs. (PR #120320)

[Kiran Chandramohan via cfe-commits]

kiranchandramohan wrote:

> > > After #120165 clang started complaining about unknown option -f[no-]realloc-lhs.
> > 
> > 
> > Was this on an existing test?
> 
> I noticed it when trying to compile a Fortran/C mix test where flang/clang were used as compilers and I passed `-fno-realloc-lhs` in flags that were used for both. Maybe it is not the right way to do it, but clang just ignored the option before my previous change.

I am guessing that this might be the case for all Fortran flags we switched over from the gfortran group to the Flang group. Previously clang could be used for forwarding Fortran options to gcc/gfortran. But this is not the case anymore.

For example, i think `ffree-form` flag would have showed similar behaviour after commit 3a1513c142f4a.

In short I think the present behaviour is OK. But if you and others feel otherwise feel free to push forward.

https://github.com/llvm/llvm-project/pull/120320