[flang] [clang] [flang][driver] Allow explicit specification of -lFortran_main (PR #78152)

[Michael Klemm via cfe-commits]

mjklemm wrote:

> In a long run, I still think that we need to get rid of `Fortran_main`. Perhaps, for the minimal, we can rename it to remove `Fortran` in the name to make people think that it is not required (by the users) to do any linking for Fortran code.

I'd agree to that!  The way gfortran does it, seems the best approach.  When there's a program unit in the code, gfortran also add a `main` definition to that translation unit.  Unfortunately, my knowledge about flang internals is not yet there, so I have no clue how to get it to emit the equivalent of the `main` in libFortran_main.  I could get away with having it in .ll file or something and pull it in from disk, but even that seems to exceed my current capabilities. :-)

I could opt-in to make this a learning project for me, but I'd need some hand holding to get it done. Volunteers? :-) 

https://github.com/llvm/llvm-project/pull/78152