[clang] [flang] [flang][Driver] Let the linker fail on multiple definitions of main() (PR #73124)

[Tom Eccles via cfe-commits]

tblah wrote:

Since this patch, I can no longer build spec2006 gromacs and calculix because they supply their own main function in a C file, then link using flang-new: leading to another definition of `main()`  from the runtime. Do I need to use a special flag to avoid linking to libFortran_main?

https://github.com/llvm/llvm-project/pull/73124