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

[Tom Eccles via cfe-commits]

tblah wrote:

> 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. 
+2
 
> I could opt-in to make this a learning project for me, but I'd need some hand holding to get it done. Volunteers? :-)
I'm happy to help, but I think we should discuss this in a community call first

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