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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> It seems like flang-new when being used as a linker with -shared included Fortran_main in the shared library. This seems wrong to me.

I am trying to recall the rationale behind that, but it's been a while :( 

Here's a relevant discussion/bug that hasn't been resolved yet. It might feel off-topic at first, but it's in fact a very similar problem - where and when should `main` be defined? It would be incredibly helpful if somebody resolved that 🙏🏻 .

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